Someone over at imgr has compiled alot of data on the Lore of Halo and the universe it exists in. Being a Halo player even I was aware of some of this stuff, so it'll be a useful read especially with some fan theories on Halo 5 and beyond.
I've copied all of the information from imgr spanning serveral pages into this one thread. Enjoy!
The Precursors
A long long time ago, in a galaxy not so far away (actually our own galaxy), EONS before the events of the Halo game trilogy, EONS before even the Forerunners, lived the earliest known race in the Halo Universe : The Precursors. Not much is known about the Precursors, but from what we do know it's easy to assert that the Precursors were the most advanced and OP super-race we know of. They were huge in stature, they lived basically forever, their technology far exceeded that of the Forerunners, and they were so advanced it'd be fair to call them Gods. The Precursors had a deep underlying motivation to create life, and thus they populated the Galaxy with countless species, uplifting planets and raising sentience. They were the Masters of the Universe, and were the mothers and guardians of all life in the galaxy - a responsibility they vaguely refer to as The Mantle. For whatever reason, the Precursors must've gotten bored or something because they began to seek a successor to themselves to hold The Mantle of Responsibility, and thus they created the Forerunners.
The Forerunners - the douchebags of the galaxy
The grand Forerunners. In the halo games we view them through the clouded spectacles of history, and see only their unfathomably majestic and grand creations such as the Halo rings and all the spectacular architecture in which much of the Halo games take place, but unlike their Godlike creators, these guys as a whole were pretty incompetent as we'll find out later on in this gallery. The Forerunners, much like the Precursors, lived absurdly long lives. Their technology and intelligence was very advanced, but they only achieved the standing of a Tier 1 civilization, whereas the Precursors were rated as a Tier 0 civilization. (I don't really know what scale this 'tier' thing is based on, but it's apparent that lower is better, and the defining jump to Tier 0 was mastery over neural-physics, aka 'magic' as far as we plebes are concerned) Forerunner society was split into rates (like a caste, but not) : Builders, Miners, Life-Workers, Warrior-Servants, Engineers, Juridical, and Theoreticals. Strangely enough, these rates were more than just job titles, each rate also had a rank in society, with some rates having far more political power than others. It wasn't uncommon for the ranking of these rates to change through political upheaval or takeover or some other factor. It happens to be that the Builder rate was in charge at the time of the Forerunner-Flood war, the firing of the Halo rings, and the complete obliteration of the Forerunners along with all other sentient life in the galaxy, so good going on that one Builders. These rates also had an internal ranking system consisting of various Forms and Manipulars. Forerunners worked a little like Eevees (Yes, that one Pokemon you stroke with the colored rocks). Forerunners begin their lives as a Manipular (Awkward Teenager) and were referred to as a 0-form Forerunner, then as he matured he would undergo rate-specific mutations and become a first-Form/second-Form/etc Forerunner with more seniority within his respective rate. Forerunners would stay within their rate for their entire lives, just like how a Jolteon wouldn't evolve into a Vaporeon. Forerunners would generally undergo 2-5 mutations over their lifespan.
Forerunner Imprinting and the creation of the Iso Didact
*The Didact and Iso-Didact actually don't come into play until much much MUCH later in the Halo universe, but they play integral roles in the upcoming events in the Halo universe, so while I'm talking about Forerunner biology I might as well throw in these two guys.* Now I wasn't kidding about the Eevee thing, when Forerunners Mutate, they do so with the assistance of a catalyst. Except instead of a funky carcinogenic stone, Forerunners mutate with the assistance of a more senior Forerunner who acts as an imprint. This imprint is generally the Mutatee's parent, but not always. During the mutation process, physical and psychological changes are made to the Manipular, and he gains portions of wisdom, knowledge, and memories of his imprint partner. There's a bunch of different types of mutation, some more dangerous than others, which all accomplish slightly different results. Through some various forms of mutation, it's possible to kind of 'copy-paste' a Forerunner consciousness into a Manipular, the resulting copy would be a manifestation of the Imprint inside of the adolescent. It is through one of these mutation processes that The Didact (the main baddie in Halo 4) who was the leader of the Warrior Servant rate, supreme commander of the entire Forerunner military, planted his seed in a young Manipular named Bornstellar Makes Eternal Lasting, thus creating a second Didact. To avoid confusion, the original Didact is renamed Ur-Didact and Bornstellar Makes Eternal Lasting is renamed Iso-Didact.
The Ancient Humans
Aww hell yeah, we humans were around wayyyy back then before the start of the Halo series as well. Turns out the Precursors used their Jedi mind powers and saw that the Forerunner were not fit to uphold the Mantle of Responsibility, so just like any respectable parent, they just birthed another child in some other part of the galaxy to succeed their legacy : the Humans - and the Precursors thus abandoned the Forerunners. The ancient humans soon became the favorite child of the Precursors, and mankind evolved extremely quickly, and at some point arguably even surpassed the Forerunners technologically. Despite all their crazy awesome technology, these ancient humans were pretty much just like us regular douchebag humans and had the lifespan of a fruit fly.
And now the Forerunners get jealous
With a new favorite child on the map, the Forerunners become quite butthurt that they would no longer inherit the Mantle of Responsibility and become Masters of the Universe as they were promised by the Precursors. So the Forerunners took the only logical route, and declared total war on the Precursors. SURPRISINGLY ENOUGH, the Forerunners actually win the war. Turns out, the Precursors suck at war despite being so vastly superior technologically than the Forerunners. The biggest factor deciding the war was that the Precursors were simply utterly shocked that one of their own creations would turn on them - and we're not talking about 'golly I didn't see that one coming' - we're talking about major psychological trauma across the entire species. The Precursors were gentle creatures who spent their days generating life, not waging war - the concept of the Forerunners turning on the Precursors was so unforeseen that it broke the Precursor's minds. After the initial strike of Forerunners against Precursors, the Precursors changed from being loving and caring gentle giants to twisted, cynical, and sadistic beings. They became insane, and begun to only create species that suffered and struggled as an outlet for their twisted psyche, and as a measure to ensure no species would ever rise against the Precursors again. Regardless of the shattered Precursor psychology, the Precursors were still only creators of life - not killers. They could not wrap their heads around violent warfare, thus lacked efficient combative defenses against the Forerunner assault.
The Precursor Genocide
Towards the end of the Forerunner-Precursor war, the situation changed from a battle to an all out extermination hunt of Precursors. The Forerunners really wanted dat Mantle. The remainder of the Precursors fled desperately to escape the fury of the Forerunners. They sought refuge in various forms, many Precursors attempted to survive by grinding themselves into a fine, desiccated powder that was intended to rebirth them some long ass time in the future. Unfortunately, the channels of time coupled with the anger, insanity, and confusion of the Precursor it was derived from twisted and perverted this powder. Instead of rebirthing the Precursors, this 'butt-hurt powder' only brought cancerous plague and death. It would later be revealed that these angry, insane, twisted remains of the precursors were the beginnings of the Flood.
First, allow me to fix a few inaccuracies and address a bit of the confusion from the first album that some members of the community have helped me out on:
1.) The Civilization Tier scale used is the Forerunner Technological Achievement Tier rating system, not the Kardashev Scale. I'm not sure where the Humans/Forerunners/Precursors would fall on the Kardashev scale because they have properties of a Tier 4 civilization such as the Forerunner use of teleportation, but none of these civilizations were truly intergalactic. Forerunner TAC: http://halo.wikia.com/wiki/Technological...ment_Tiers Kardashev Scale: http://en.wikipedia.org/wiki/Kardashev_scale#Type_IV
2.) As a few Imgurians pointed out, it is possible for the Forerunners to change between rates, but such a transition was exceedingly rare. The most prominent example would be many members of the Warrior-Servant rate transitioning to the Builder rate following the exile of the Didact after losing a political battle against Master Builder Faber.
3.) The Forerunners aren't quite as mean as I made them appear. As it turns out, the Precursors - while not violent, weren't particularly loving either. They would experiment and create life, and after a failed experiment there's precedent that they might erase the failed experiment, aka exterminate the created species just like a scientist might put down lab rats that had cancer. The Forerunners had suspicion that the Precursors might delete them because they were not fit to uphold the Mantle, and therefore attacked the Precursors although there was no evidence that the Precursors intended to erase the Forerunners. The attack was thus basically unprovoked, and the ferocity of the uprising astonished the Precursors. The Forerunners weren't Nazis either, they felt pretty bad about turning on their masters and the whole genocide thing. Towards the end of the War the extermination crews sent to the Small Magellanic Cloud to kill the remainder of the fleeing Precursors actually committed suicide in shame after completing their mission.
4.) So far the story is being told in a roughly chronological order. The events thus covered have ranged from 1Million BCE to ~ 100,000BCE. 100,000 BCE is when the Halo Array was first fired, destroying all sentient life forms in the galaxy. The events of the Halo game series don't occur until ~2500 AD, so that's why there's been no mention of the Covenant/UNSC
The evolution of the Flood
So when we left off last time, the Precursors were being hunted down and exterminated by the Forerunners, and some Precursors had ground themselves into a desiccated powder to hopefully reincarnate them at some later time, but time and the Precursor's insanity had perverted this butt-hurt-powder into the proto-flood. Now, much much later at around 110,000 BCE, the Humans discovered ancient derelict ships containing vials of this infectious powder. The Humans had no idea what this dust was, and early experimentation showed that this power induced slight psychotropic effects in lower life forms. Being the smart creatures that we are, instead of putting the powder back where it came from, the Humans made the very logical decision to start applying the butt-hurt powder to their pets, primarily the Pheru which was like a space-dog (but not), because it improved domestic behavior. These powdered Pheru became such great pets that the San'Shyuum (AKA The Prophets from Halo) who were the allies of Ancient Humans also began adopting Pheru as pets - spreading these powered Pheru even further. This practice of getting your space-dog high off strange powder found in deserted spaceships carried on for hundreds of years, and all the while the butt-hurt-powder spread within the space-dogs, altering its genetic code and passing the mutations from generation to generation of Pheru. Somewhere along the line, these Pheru started displaying physical signs of change. At first, it was a small patch of loose, soft fur between the animals shoulder blades which was probably silky smooth to the touch. Other Pheru consumed this fur, and sometimes the entire animal as well which was odd because Pheru were herbivores, but of course this still didn't trigger any particular alarm in the Humans. This did, however, trigger a biological timer within the powdered Pheru because within a short period of time the Pheru begun to take on really ugly physical traits including horns and tentacles, which continued to be consumed by other Pheru. The consumption of these unattractive growths lead to mutations, abortions, and unnatural births within the Pheru.
So now the Humans start worrying...
Now that their domestic Pheru started getting real ugly, the Humans began to suspect that drugging their pets with strange unknown dust harvested off mysterious abandoned foreign spaceships might've had some bad side effects. Unfortunately, the damage was already dealt : the Flood had spread to humans. I find this cross-species jump a bit strange, because the Flood has only been able to pass its mutated genetics to another creature through consumption of body parts, so this means at some point some guy ate his dog. Well anyway, these infected Humans displayed symptoms similar to the infected Pheru in that they had an altered state of mind, had an insatiable desire to consume infected Pheru flesh, and were unimaginably ugly. These afflicted Humans soon started spreading the infestation by force feeding non-infected victims with infected limbs and various other body parts until they grew to gross proportions, and then proceeded to eat the land-whale they produced to further gestate the Flood infestation. Soon enough, the Flood evolves into the parasite we're familiar to seeing in the Halo series.
The Human-San'Shyuum Alliance - Flood War
The Humans/San'Shyuum, who have up to this point proven themselves to be true masters at the art of risk-assessment, go full panic mode and apply "Scorched Earth" tactics to combat the Flood. The Alliance torched huge sectors of friendly galactic space, earning a pyrrhic victory over the flood on each infested planet they utterly destroyed. Unfortunately, when you toast hundreds of your own planets, you naturally run out of space to live. As the Flood pushed back the Humans/San'Shyuum, the Humans/San'Shyuum pushed into Forerunner Territory in desperate attempts to replace planets lost to the Flood as they fled. Apparently, nobody told the Ancient Humans that beggars can't be choosers; Foolishly, the Humans, as they fled, were arrogantly purist and cleansed 50 defenseless Forerunner planets for themselves to inhabit. (If you've watched the Halo Terminal videos, you get might the impression that the Humans only purged Forerunner planets in a well-meaning but ill-fated way to combat Flood infestation, but make no such mistake - the war initially broke out because humans were assholes didn't believe in sharing, which is why they 'cleansed' these 50 planets) While the inhabitants of these Forerunner planets that the Humans killed weren't actually Forerunners but rather various lower species that the Forerunners had relocated as part of their never-ending quest to fulfill the Mantle, the Forerunners were heavily butt-hurt by this offensive and thus began the Human-Forerunner war.
So why didn't the Humans and Forerunners just negotiate?
(^ That guy's visor makes him look so sad Maybe he's re-evaluating his life choices) The galactic butt-hurt is real. The analogy to draw here is that Humans were the annoying, entitled, and spoiled younger child, while the Forerunners were the bossy, know-it-all, wanna-be-mature older sibling that may or may not have killed his parents. After the Precursors spawned the Humans, the Humans made incredibly fast technological progress, fueled by an abundance of Precursor artifacts to reverse engineer. By 150,000 BCE, the Humans were a Tier 1 civilization (On the TAC scale, not Kardashev) - on par with the Forerunners. The Humans soon encountered the San'Shyuum (The Prophets) and formed an alliance centered around the planet Charum Hakkor which will become a pretty important hub for future events. Being the self entitled children they were, the Humans resented being told what to do by the Forerunners who had self-proclaimed themselves as bearers of the Mantle and as such went around bossing around everyone in the galaxy. To get away from the Forerunners, the Humans expanded far away in the Orion Arm of the Milky Way galaxy, and grew to the strength of more than 20,000 planets across thousands of star systems. While the 2 races mostly ignored each other after Humanity went off to play in its own corner, the Humans and Forerunners fundamentally hated each other because both felt entitled to succeed the Mantle after the 'unfortunate' passing-away of mommy Precursor, but of course "THERE CAN BE ONLY ONE". As such, the two races saw each other as bitter rivals, and each marked the other as 'dirty, godless Heretics'. (Where have I heard this before?) With this much butt-aching tension between the races, it's no surprise that the Humans didn't really issue a 'head's up' to the Forerunners as the Humans advanced into Forerunner space and cleansed 50 of the Forerunner's planets, and it's even less of a surprise that the Forerunners didn't have to think twice before declaring total war.
Humanity's struggle
At this point, Humanity is basically Nazi Germany in space - they were fighting a 2 front war against the Forerunner 'Allies' on one side, and the Flood 'Russians' on the other side while performing genocide the poor species who got caught up in the conflict. The Human military, lead by Lord of Admirals Forthencho, stuck between a rock and a hard place, prioritized purifying flood infestation over fighting the Forerunners - whether it be on a Human or a Forerunner planet. When a Forerunner planet became infected, Forthencho's immediately nuked the crap out of it, killing millions of Forerunners in the process. (This would be when that scene in the Halo Terminal video where Forthencho solemnly orders the purging of an infected Forerunner planet took place) The Forerunners at this point in time rated Humanity as the single biggest threat in the Galaxy, and blew off the Flood as 'not a big deal' and 'we can deal with them later easily'. Because of this, the Forerunners saw the Human methods of purification of the Flood on Forerunner worlds as an egregiously heinous act of violence, and a complete affront to the Mantle. The Forerunner wars lead by the Didact, the leader of the Warrior-Servant rate and the supreme commander of all Forerunner military, with EXREME belly-aching anger towards the Humans who he essentially calls "baby murderers", adopted a planet-hopping strategy against the humans in which he jumped his ships directly to important Human worlds to destroy them, ignoring the more trivial planets - efficiently back-dooring vital human resources and power hubs.
The Primordial
MEANWHILE, through all this conflict, Humanity discovers a giant, ancient, mysterious life-form sealed in stasis, and transports it to Charum Hakkor. This life-form turns out to be The Primordial, who is as far as we know the last surviving Precursor (that isn't ground up into butt-hurt powder anyway). The Humans don't really know what it is, but of course start performing experiments on it because that went so well the last time they dug stuff out of space. Through a good amount of experimentation, the Humans gain the ability to rudimentarily communicate with the creature, and scientists start asking the Primordial the big questions like, "Why are we here?", "How did we get here?", "Why are the Flood attacking us?", and "Why are the Forerunners so butt-hurt?" The Primoridal answered these questions with a truth so terrible and disturbing that it lead most of the scientific team to commit suicide. The Humans attempted to cover up this information, but despite their efforts, word of the Primordial's truths got out demoralized the Humans drastically - as you might imagine.
Humanity's 'victory' over the Flood
Very surprisingly, through all of these terrible events that the Humans definitely didn't bring upon themselves, the Humans actually managed to defeat the flood by purposely infecting 1/3rd of their population with a genetically modified version of the flood and placing these special Flood-infected humans in the incoming path of the Flood onslaught. Fighting fire with fire apparently worked, because the Flood assault was stopped in its tracks, and the modified Flood forms aggressively attacked the original Flood and drove them into hiding for 9000 years. It would later be revealed that the Flood 'let' the humans win (maybe they did, maybe they just got rekt) because whatever remains of the original Precursors inside the Flood found that the Humans were worthy of upholding the Mantle. This yet again pyrrhic victory over the Flood was short lived, because during this whole time that the Humans were fighting off the Flood, the Forerunners were back-dooring the Humans.
THE PRECURSORS:
1.) Originally I implied that the Precursors weren't truly inter-galactic because we mostly just hear of them staying in the Milky Way, but it turns out that it's possible that the Precursors were vastly inter-galactic and either chose the Milky Way as their final resting place, or are just out in space somewhere in other galaxies that we don't hear about.
2.) The Precursors created many, if not all of their races with the expectation that they might be worthy to succeed the Mantle; The Precursors would create a species, then observe their development over vast periods of time to determine whether or not they were worthy of the Mantle. This would mean Forerunners and Humans weren't particularly special, but as far as our story is concerned, they are the best choices to inherit the responsibility for all life.
3.) Not all Precursors look the same - they're capable of taking on multitudes of physical forms with the angry powder being one of them apparently. Thus it could be speculated that there are some other dormant Precursors in the Milky Way, but none of them do anything that we're aware of in our chronology.
THE MANTLE OF RESPONSIBILITY: The Mantle is a general term for having the responsibility of taking care of all life in the galaxy. It's somewhat of a vague concept because who's really to say 'taking care of' means? The Precursors have shown themselves to create and erase species at will, so good vs bad is somewhat of a grey area. Regardless, the Didact has implied that during the time the Forerunners upheld the Mantle, they were 'Guardians of all that exists, the careful tenders of the roots of the galaxy, the luminous sun towards which all life and intelligence blossoms, the impervious shelter beneath which all life has prospered'. (< He might be slightly biased though) THE FORERUNNERS DIDN'T KNOW ABOUT THE FLOOD DURING THE FORERUNNER-HUMAN WAR: 'Humanity is the most contentious, bigoted, and self-centered species in the galaxy' - The Didact, Supreme Commander in Butt-hurt of Forerunner military forces. This is kind of a questionable area. While some sources suggest that the Forerunner had no clue of the parasite, IMO it's a safe assumption that the Forerunners knew damn well about the Flood, especially since as holders of the Mantle and defenders of all Life, they should really be aware of all life in the galaxy or else they'd be some really bad parents.
Even if they didn't know about the Flood at the start of the fighting, they certainly would have realized it at some point during the very very very long period of conflict with Humanity. At the end of the war when Forthencho surrenders to the Didact and right before the Forerunners blow humanity back into the stone-age, the Didact even says something along the lines of 'Dear Lord of Admirals AKA killer-of-our-children : soon we'll face the Flood too, and we're scared'. It just happens to be that the Forerunners and particularly the Didact don't like the infant-slaughtering Humans - and overall the Forerunners rated Humanity as a far greater threat to the Mantle than the Flood which was still in its earliest stages.
HUMANITY'S VICTORY OVER THE FLOOD: It's revealed that the Flood never wanted to attack the humans, but rather they were after the Forerunners and the humans just happened to be in the way. This might be because the Humans were fit for the Mantle, this might just be because the Precursors didn't like the Forerunners. When the Flood fled like lilly-livered scaredy cats from humanity's 'cure', its revealed that this act of cowardice was actually a 'tactical retreat'(get rek't m8) - AKA all part of the greater plan to destroy the Forerunners. This does make sense because as the Flood gain biomass, they not only increase in military force but also intelligence. While I'm not sure whether or not a Gravemind had been created during the Human-Flood war, its a fair speculation that at some point the Flood became smart enough to realize Humans would either die to the Forerunners or manage to defeat the Forerunners - either way it'd be best for the Flood to simply chillax for a few thousand years while the Humans and Forerunners duked it out.
Continuing on the Human-Forerunner War
So the 2 fronted War of Humans vs Forerunners and Flood was a little like rush hour on the Subway where the Flood were pushing up against the Humans rear, and the Humans were like 'get the fuck out the way' to the Forerunners in front. While the Humans were getting rear-ended by the Flood for the majority of the thousand years of confilct, The Forerunner back-doorers lead by The Didact were also incredibly efficient in their "Star-hopping" strategy against the Humans. Forerunner battle groups would jump past trivial Human territories and go straight to vital Human power centers and destroy them. Erde-Tyrene, aka Earth - the home planet of the Humans - was one of the first planets to be defeated by the Forerunners. Losing their homeworld to Forerunner back-dooring strategies was not only a crushing blow to morale as you might imagine, but also an indicator of how heavily stacked the odds were against Humans. Humanity's forces were spread thin and sandwiched between Ugly and Uglier, and despite the Flood overtaking massive sectors of Human space, Humanity's forces never pushed within 15,000 Light-Years of the Orion Complex - the center of power for the Forerunners. As much as life sucked for Mankind right now, Humanity against all odds actually managed to drive back the flood who were all like '#plannedRetreat'. During the Flood's clever tactical retreat, they still held thousands of worlds, and it was during this period that Forerunner forces, presumably during one of their honorable back-dooring missions, encountered the Flood, resulting in losing hundreds of battle fleets to the Flood. At this point, the Forerunners who had always underestimated the flood, realized the huge danger the parasite posed. The Forerunners, in their infinite wisdom and absolute despise for the baby-killing battle tactics of the humans, adopted the baby-killing battle tactics of the humans and utterly torched any planet found to host the Flood parasite. It was also during this time that the Forerunners, through experimentation on captured flood samples, learned of the Flood's 'logic plague' which had corrupted Forerunner Anscillas on board the infected Forerunner fleets, who were AI's much like Master Chief's Cortana.
Humanity's surrender at Charum Hakkor
Humans had driven back the Flood, and with the full force of their remaining military, actually managed to deal crippling damage to the Forerunners as well. Unfortunately, the Humans were simply too battle-worn at this point, and it wasn't before long that the Forerunners managed to surround and cut off Forthencho's forces at Charum Hakkor. Of course Forthencho didn't just roll over and die : Charum Hakkor became the space-Alamo of Humanity. Over the period of almost 50 years, Forthencho's forces managed to beat off the Forerunner offensive despite being cut off from reinforcements through sheer will-power and the power of love (Not really) As it turns out, aside from being the center of the Human-San'Shyuum Alliance, was also a hub of Precursor technology - including a good amount of Unbending Filaments which were huge neural-physics cables spanning several kilometers thick constructed by the Precursors as a means of extremely fast travel between planets and star systems. By incorporating Precursor artifacts into their defense, the humans for 50 years were able to drive back even the most formidable of foes. It was during these 50 years that the leader of the San'Shyuum , who had been allies of the humans up till this point, surrendered to the Forerunners in hopes of a lighter sentence - stranding the remaining Human and San'Shyuum forces on Charum Hakkor to die. Just as the Alamo eventually fell, the human battle cluster at Charum Hakkor also eventually fell to the Forerunner offensive. A large proportion of the defeated Humans and San'Shyuum opted to commit suicide than to surrender to the Forerunners - a wise choice as we'll soon find out.
Aftermath
Humanity and Forthencho - who like a real good captain didn't kill himself and go down with the ship, submitted to absolute 0-conditions surrender to the Forerunners. Forerunners, being the generous folks they are, decide to only wipe Humanity's space-faring presence off the face of the Galaxy, completely destroy all existing Human civilization, and toss the surviving Humans back on Earth and devolve them into the banana-munching apes of the stone age. As you might infer, there weren't a lot of remaining Humans if an empire spanning 20,000+ planet's survivors all fit back on Earth. It actually turns out later the Forerunners actually weren't as generous as their very mild punishment for Humanity would have made them appear. The Forerunners had originally planned drive Humanity into extinction much like all other races who dared challenge the Forerunner's rightful hold over the Mantle that they acquired 100% legitimately, but the pleading of the the Librarian saved Humanity from utter destruction. The Librarian, the leader of the Life-worker rate, and wife of the Didact, and a human sympathizer, convinced the council that Humans had a cure to the Flood which was the reason why the Flood suddenly withdrew during the conflict. Though the Librarian herself was very doubtful of the existence of a cure, she lied through her teeth much like your Mom might defend you in a parent-teacher meeting and managed to spare meager remains of Humanity from death with the promise that Humanity would certainly share their secrets of victory against the Flood with their conquerers ... KIND OF. The Forerunners must have been very butt-hurt, or suspected that the Humans were very butt-hurt, because rather than allow the Humans to live happily ever after on Earth in their biological forms, many of the surviving Humans on Charum Hakkor had their minds painfully extracted and saved in a digital format through the use of the Composer which you might have heard of from Halo 4. The Forerunners probably felt justified in their assumptions of Human belly-aching, because despite thousands of years of interrogation, the Forerunners weren't able to extract a plausible cure to the Flood out of the composed Humans. Somewhere along the lines of these thousands of years of painful interrogation, many Life Workers adopted the Librarian's view that a cure for the Flood didn't actually exist and that the Flood had retreated for other reasons. (Not that this stopped torture interrogations.) The San'Shyuum on the other hand, received a much lighter sentence than the Humans. Maybe because only fighting against the Forerunners for 950/1000 years instead of the full 1000 didn't merit the same punishment as humanity, or maybe the Forerunners didn't hate the San'Shyuum as much. The San'Shyuum, rather than face extinction or de-evolution, were simply confined to their home world of Janjur Qom. Access to the planet sealed off through quarantine-shields, and this would have almost been a decent happily ever after, but the San'Shyuum soon face a pretty ugly fate later on in the Forerunner-Flood war.
BASIC TIME-LINE OF EVENTS SO FAR
Billions and Billions of years BCE : The Precursors show up in the Milky Way and seed a bunch of species, including Humans and Forerunners. I say 'seed' because the Precursors weren't exactly like God who just straight up plopped out Adam and Eve, they simply set up the conditions for a race to evolve naturally. IMO this is pretty cool to think about because it really brings legitimacy to the Halo universe in that it *could* be our real universe. 500 Million BCE : The Precursors create the Organon. This thing is super super important and there will be more about this later 15 Million BCE : The Forerunners are 'created/seeded' 10 Million BCE : The Precursors determine that Humans should carry the Mantle. The Forerunners revolt and exterminate the Precursors 1.1 Million BCE: The Ancient Humans finally get the rocks out of their asses and start becoming a relevant interstellar species ~110,000 BCE : The Ancient Humans discover the proto-flood powder ~109,000 BCE : The Humans lose the Human-Forerunner War
CLARIFICATION ON THE PRECURSORS
*DISCLAIMER* A lot of what will be said is actually from personal speculation, but it is based heavily off of established resources. I welcome any community discussion or theories on this section*
I've greatly understated the Precursors in my previous posts - it turns out for the Halo Universe, the Precursors fulfill the roles of literal Gods and Angels. And I mean do mean literally. The fact is the Precursors are so incredibly old that they could have existed from the dawn of the universe and they're so unbelievably advanced they've transcended the need for physical forms and exist on an abstract level, much like their neural-physics creations. They can take on any form they choose, thus we can't really be sure what they are, where they are, what their intentions are, or what they're doing because they could be in any physical or abstract form.
The Precursor's technology of neural-physics is linked closely with their 'theology' (I put theology in quotes because the Precursors certainly weren't wrong as far as the Halo Universe goes, or else their technology wouldn't work) The Precursors saw the Universe as a living entity, and that the Universe gains great joy and enrichment from the experience and interactions of life with the cosmos - and as such the Precursors created the concept of the Mantle. The Precursor Mantle however encompassed a whole lot more than regular life, it decreed that all existence : life, energy, matter, and even the universe itself fell beneath the responsibility of the Mantle.
In the Halo Universe, the Precursors take on many of the roles deities perform in religion : they are the genesis for life, they are the definitive factor for 'salvation' or 'damnation' for species, they operate in mysterious ways, and they are utterly incomprehensible. I think the cleanest way to think of this is with the Universe as 'God' and the Precursors as its 'Angels' - except there's not really a 'heaven' or 'hell' nor 'Angels' vs 'Demons' - they all just fulfill the will of the Universe in different ways. This would also explain why the Precursors, the hands of God, are the only race capable of utilizing neural-physics which allows them to construct miraculous constructs that tap into the knowledge and experience of the Universe and to manipulate the very fabric of existence. While there's no 'Heaven' or 'Hell', there are certainly ways to piss off the Precursors. The Forerunners, who in their jealousy of their neighbor, Humanity, rose and killed their Precursors creators, were sentenced to damnation for their transgressions against the Precursors by means of the Flood (omg y'all see that Biblical connection?)
SO THAT LEAVES A FEW BURNING QUESTIONS...
*DISCLAIMER: These answers are almost purely opinion*
1.) Did the Forerunners really wipe out the Precursors? ... HELL NO. At this point I'm not convinced the Forerunners managed to 'kill' even a single Precursor, or whether that's even possible. It's likely that the Precursors are so integrated with neural-physics and the Universe that they can alter forms and transition between matter and energy at their utter discretion - getting 'shot' by Forerunner weapons would just disperse a Precursor into some other type of matter or energy.
2.) Why don't the Precursors just come out and set things straight? Why don't the Precursors just come back to the Milky Way? Why don't the Precursors just show themselves? Why don't the Precursors hear my prayers? Why don't the Precursors bring me a damn Bugatti? Precursors work in mysterious ways, and much like regular God, probably don't give too many fucks about micro-managing everything even if you ask very very nicely. I think the Futurama image describes this situation perfectly.
3.) Why are the Precursors looking for successors to the Mantle? I actually don't know, but we can be damn certain that the Forerunners did NOT receive the true Mantle because to actually inherit the Precursor Mantle would imply mastery of neural-physics and most certainly the ability to monitor everything in the Universe including all forms of energy and matter, not just living entities in the Milky Way galaxy. Additionally, we know the Precursors are intent on punishing the Forerunners. In my opinion, the Precursors could even be putting the Forerunners in their own version of Hell in which they THINK they're the holders of the Mantle, but are unable to fulfill their duties to the Mantle despite all their efforts, and ultimately destroy all the life they were supposed to safeguard. Through such struggle and pain, Forerunners would truly realize their mistake and see that they are not fit to uphold the Mantle.
4.) What do the Precursors truly look like? The Forerunners speculated that the Precursor created Forerunners and Humans in the image of themselves, but I don't know how much the Forerunner's account can be trusted because first, a whooole lot of species look like they got 1 head 2 arms and 2 legs, and secondly the Forerunners were a bit silly.
THE FORERUNNERS
I know we've done a whole lot of bagging on the Forerunners, but the Forerunners were truly an amazing race worthy of our respect - it's just kinda unfortunate that they get the short end of the stick on almost everything. They are the tragic-hero of the Halo Universe; They are ultimately good natured but have crippling downfalls and weaknesses. The Forerunners were created by the Precursors along with a bunch of other races when the Precursors first gave life to the Milky Way galaxy. The Forerunners were seeded on the planet Ghilbalb in the Orion Complex and Ghilhub served as the hub of the Forerunner's early interstellar civilization which spanned 12 nearby star systems. Unfortunately, when these early Forerunners experimented with star manipulation, they accidentally triggered a series of supernova explosions that rendered the entire region uninhabitable and nearly wiped out the Forerunners. Fortunately enough for the Forerunners and our story, they actually didn't all die and after that 'little hiccup' the Forerunners progressed into a highly advanced interstellar species. As they grew, the Forerunners came to suspect that the Precursors would soon come and judge them as to whether or not they were fit to inherit the Mantle of responsibility for all Life. Unfortunately for the Forerunners, around 10 Million BCE the Precursors determined that it would be Humanity that inherits the Mantle. The Forerunners, struck with jealousy and maybe a sense of betrayal by their creators, rebelled against the Precursors and 'exterminated' them. The Forerunners, despite their uprising, were extremely ashamed of their rebellion and tried in many ways to cover up the Forerunner-Precursor war. Apparently these cover-ups worked because in time, future generations of Forerunners simply came to believe that the Precursors simply got up and left after successfully creating their successor : the Forerunners.
In the following eras, the Forerunners had a pretty rough time getting their shit together. They were constantly engaged in civil wars, political turmoil, and social strife. The Builder rate even attempted to purge other rates such as the Warriors and Miners - and actually managed to annex the Theoretical rate into the Builders. While the Forerunners never quite became less contentious, they eventually leveled out and grew into dominant species in the Galaxy we know them as in about 150,000 BCE. At the height of their power, the Forerunner Ecumene AKA Empire spanned over 3 million fertile worlds in the Milky Way galaxy and created it's central command in the Orion Complex - in comparison Humanity at the height of its power controlled only 20k-30k worlds. Now that doesn't necessarily mean there were a ridiculous number of Forerunners in the galaxy and that they got real 'busy' like rabbits; the Forerunners likely claimed control over any planets inhabited by lesser species under the rite of the Mantle - and many planets actually inhabited by Forerunners are sparsely populated. It's not uncommon for a wealthy Forerunner family such as that of Bornstellar Makes Eternal Lasting (that one guy who later becomes the Iso Didact) to own an entire planet to themselves - thus the actual number of Forerunners in the galaxy would be a lot lower than you'd expect from an empire of 3 million planets.
To understand the Forerunner's actions which may seem a bit weird to us, it's important to understand their mindset. The name 'Forerunner' is a translation of the race's name for themselves. They called themselves Forerunners because they believed they were the first race to succeed the Mantle from the Precursors - and that in due time many other superior races would succeed the Forerunners as protectors of the Galaxy. I speculate that this is the part of the reason that the Forerunners were frankly kinda suicidal and wayyyy too chill with the concept of a dark-romantic extinction. If someone had approached the Human Empire with the concept of the Halo array, you can be damn certain Humanity's response would've been, "Aww hell naw". At most, Humanity would have used the Halos as the last resort 'Fuck-You' button that the literal last-man-standing in the entire species would have pressed on his dying breath. But of course the Forerunners were like, "Halo Array? Sounds great to me". The impermanence with which the Forerunners viewed themselves is also why, after the firing of the Halos, the Forerunners don't come back - instead the Forerunners pass on the Mantle to Humanity and in turn rename Humanity as the Reclaimers. Yes, finally we get a cool sci-fi name!
THE UR-DIDACT AND THE PROMETHEANS
Not all Forerunners were chill with fading away. This is why the Didact is so controversial to Forerunner society. He's fiercely devout to the Mantle, has an undying love for the Forerunners, and believes the Mantle belongs solely to the Forerunners. The ending of cinematic of Halo 4 does a good job of summarizing his sentiments (In the Video he's talking about modern Humans rather than Ancient Humans and the premise is a little bit different, but it represents his views pretty well)
At the time of the Flood-Forerunner war, the Didact and his supporters heavily opposed the construction of the Halo Array proposed by the Builders, and ultimately the Didact was exiled for his opposition and the Warrior-Servants were marginalized because a good amount of Forerunner society supported the concept of Halos and also because the Builders are douchebags.
THE ORGANON
The Forerunners mistakenly believed that the Organon was a universal key to activate all Precursor artifacts; As such, they valued the Organon as the most sought after Precursor creation, but searched for it in vain because unbeknownst to the Forerunners, the Organon was actually one and the same with what the Forerunners called 'the Domain' which was like 'the voices of their ancestors' in their heads that I guess they just thought was part of their physiology. It wasn't until very late in the Forerunner-Flood war that the Forerunners discovered the truth. The Organon was actually a Precursor neural-physics construct that compiled and indexed all Precursor knowledge encompassing hundreds of billions of years into a unified reservoir and embedded inside of ancient Precursor artifacts distributed throughout the the Milky Way(?). Sources are unclear as to the size and scope of the Organon - it may have been a specific construct for the Milky Way galaxy or may be part of a larger universal knowledge network. IMO it was probably like portable hard drive with a cloud connection to the main network. Regardless of the scope, the Organon is closely related with the Precursor's ability to 'communicate' with the Universe entity, and it was capable of interacting with and activating many - if not all - Precursor artifacts. Due to its neural-physics structure, you could access it with your mind just like the Jedi would. The Forerunners were likely the most skilled species other than the Precursors at utilizing the 'jedi mind powers' but they were still merely Padawans and found the Domain blurry and confusing, much like everything Yoda says.
The Forerunners suspected that the Domain was a self-aware entity because the Domain would often change information within itself "like real memories", hide away secrets, and had a tendency to inform the Forerunners when they acted against its will. Late in the Forerunner-Flood War, the Flood gained control of the Organon and through it a great deal of Precursor artifacts that they Flood used to close out the war. Sooo in conclusion, think of the Organon as like, a distributed operating system for the Precursors. It had the ability to store information, make changes, protected user privileges and accounts (e.g. a Forerunner wouldn't be able to access the memories of a Precursor through the Organon), and was capable of running Precursor 'applications' AKA artifacts.
Source: http://imgur.com/gallery/bI6Ih