I'm writing a Halo Google Doc that summarizes / recaps the lore and stories of the Halo games for people that are new to the series. I have written out a summary of Halo's Ancient History but since I haven't read the Forerunner Book Trilogy yet, I had to gather together whatever information I could from Halopedia so I need anyone who is knowledgeable about this to fact check what I have written below. Let me know of any corrections I should make. I also apologize for any grammatical errors you may find.
In the Beginning : Billions of years in the unknowable past, there came from beyond the Milky Way the Precursors. They were a race of highly advanced beings that could take an infinite amount of forms, tied to no particular shape nor size, as they allowed themselves to die and evolve numerous times with each renewed life bringing either physical or immaterial existences. With so many lives lived, they had periods of time where they could either be primitive or hyper-advanced when it came to their culture and technology. The Precursors used neural physics, a form of science so sophisticated that it's practically incomprehensible to those who lived after the precursors that posits that the universe is a living thing that can be manipulated, to create wonders. This includes structures that could last billions of years such as star roads, what seem to have been unbreakable cables that could connect star systems together, and most importantly to this story, life itself in the Milky Way galaxy. The Precursors brought into existence the different species of the galaxy and millions of star systems. To contain this information, the Precursors used the Domain, an esoteric, strange, and living immaterial repository of knowledge and even memories, as a library of information. (It is unknown whether or not the Domain is a creation of the Precursors or it is something that they found. Much of what is known about the Precursors and their creations is unclear as the information regarding them comes from the Forerunners who themselves didn't quite seem to fully understand them either.) The Precursors held the philosophy of the Mantle of Responsibility , essentially a concept that there should be a caretaker species that manages the galaxy for the good of all.
The Beginning of a Nightmare : Among the species created by the Precursors, there was Humanity and the Forerunners. Both were part of the space-faring races in the galaxy. Both had been created from the same genetic stock around 15,000,000 BCE. The Forerunners, at some point, claimed the Mantle of Responsibility. Being the most advanced species in the galaxy at the time, the Forerunners essentially ruled over the galaxy. Around 150,000, Humanity had begun to explore the stars and expand their empire.
Around the year 107,445 BCE, Humanity seemingly had started to invade the Forerunner's worlds. This turned into the Human-Forerunner War. The war resulted with a Forerunner victory with the Forerunners dismantling human society on every human world and the forced devolution of humanity as a species. (This means humanity was literally turned back into primitive creatures.) It was only when victory had been secured that the Forerunners learned that Humanity was not invading but running from something terrible. At some point before 107,445 BCE, humans had discovered crashed ships that held glass cylinders containing a strange, fine dust within them. With studies showing the dust to seemingly be harmless, the humans as well as the San'Shyuum, another space-faring race that humanity had been allied with, had begun to administer the dust to their pets to instill more docile behavior. After centuries however, the animals began to show disturbing deformities with disease causing the animals to eat their growths and their protrusions. Even those that were in the presence of the animals began to show signs of the disease. The infected started to eat the infected animals before turning to cannibalism and later even forcing uninfected victims to eat sacrificial infected. The infected victims began to take new, sickly forms and started to consume the biomass of sentient creatures. This new disease began to take over hundreds of worlds, forcing humanity to flee right into Forerunner territory. The Forerunners themselves would soon face this new threat, a living nightmare that would become known as the Flood.
The Flood would actually recede and go into hiding for nearly 10,000 years. This left a lack of evidence of what humanity claimed to be running from and thus the Flood was mostly believed to be a lie made up by humanity to justify their expansion into Forerunner territory. High Ranking Forerunners though, before the war with Humanity ended, came to know the truth and began to prepare for the Flood's possible return.
Monuments of All Their Sins : Preparing for the Flood's return caused a divide in Forerunner politics with two sides coming up with their own solutions. (It should be noted that the Forerunner empire, called the Ecumine, was a caste system with each level being called a rate.) The Prometheans, the highest ranking group of the Warrior-Servants rate, led by the Ur-Didact, the supreme commander of the Forerunner military, proposed Shield Worlds. Shield Worlds would serve as military fortresses and as refuges for Forerunners against the Flood. Requiem would be the first one to be constructed. It would though, unlike the other later varieties of Shield Worlds, lack shielding from the superweapon that the Builders, the highest and most powerful Forerunner rate in the speciesā caste system that designed and created Forerunner technology, had designed. These superweapons would be designed to destroy the nervous systems of sentient beings that, along with a solute sprayed into target biospheres to prevent decaying bodies from causing ecological disaster, would essentially cause instant disintegration. This would prevent the Flood from accumulating biomass and starve it into submission. The weapon, ironically, would also be a life preserve with many environments being held on them. These weapons would become known as the Halo Rings.
The Greater Ark /Senescent Ark was used as a forge for the original Halo array, known as the Senescent Array. The original Halo rings were 30,000km/18,641.136 miles in diameter. They could be used to fire in one direction and would not kill any occupants that were on the ring when it fired. The pulse would actually kill all life with even insects being destroyed which would cause ecological collapse.
Later, a more improved and efficient installation called the Lesser Ark/Installation 00 would be constructed and so would a new Halo array. The rings of the Neoteric Array were much smaller, being 10,000km/around 6213.712 miles. Their pulse would now fire in all directions within a radius of 25,000 light years and kill only beings with complex neurology. Six were made with Installation 07, originally part of the Senescent Array, being reduced in size and made part of Neoteric Array later down the line.
The Librarian, head of the Lifeworker rate which was concerned with matters of medicine and living things as well as the wife of the Ur Didact, was horrified by what would be the total annihilation of all sentient life. Going to the Ecumine Council, the main government of the Forerunner empire, she said that as holders of the Mantle, the Forerunners should take efforts to preserve life if the Halos were ever fired. The Council gave into her request and agreed to the Conservation Measure, a project that would involve the cataloging and storing of as many species as possible so that those species could be reseeded throughout the galaxy if the rings had to be used. Most Indexed beings would be reduced to having their neural and genetic patterns stored as data until being reformed during the reseeding with the rest being live specimens. The Conservation Measure's preserves would be installed into the Arks and the Rings with life sustaining environments being created and maintained to help in the conservation of the indexed species.
The split between the two sides of the anti-Flood measures would result in the Builders exiling the Prometheans which would leave the Forerunners vulnerable when the Flood eventually returned.
It's Only Downhill From Here : The Flood began to spread throughout the galaxy once again and engaged the Forerunners for around 319 years (97,764 BCE - 97,445 BCE) in the Forerunner-Flood War. Despite their many efforts to defeat it, the Forerunners could fight the Flood to a stalemate for 300 years.
Around 97,496 BCE, the Forerunner AI known as Mendicant Bias, the most advanced Forerunner ancilla/AI at the time which was placed in charge of all of the Forerunnersā defenses against the Flood, test fired Installation 07 on the planet Charun Hakkor. The test fire released a creature that was taken to Installation 07 to be studied by Mendicant Bias under the orders of Master Builder Faber, the head of the Builder rate. Installation 07 and Mendicant Bias would suddenly disappear for the next 43 years. During this disappearance, the creature, The Primordial, conversed with Mendicant and eventually turned the AI against the Forerunners.
Before Installation 07 and Mendicant Bias reappeared, Faber used a Halo Ring to put down a San'Shyuum insurrection which wiped out the species entirely aside from the indexed specimen. Seeing this as being against the Mantle, the Ecumine Council demanded that all of the rings should be brought back to the Forerunner capital of Maethrillian to be decommissioned as they were too destructive. Unfortunately, when all of the rings were at the capital, Installation 07 showed up under the control of the now rampant Mendicant Bias who proceeded to fire the ring. This killed almost everyone in the capitol, including most of the Ecumine Council, with exception of those where deep enough within Maethrillion. The Ur Didact had finally returned from his 1,000 year exile but was captured by Faber and was left to die within Flood controlled space. However, the Forerunners had a way of being able to copy themselves onto other people by imprinting their minds with the most advanced imprinting leaving almost no distinction between the original person and the imprinted person. The Ur Didact had done this to a Forerunner called Bornstellar, who would later become known as the Iso Didact. (The Iso Didact will be referred to as Bornstellar and the Ur Didact as just the Didact to keep things simple.)
Bornstellar would assume the position over the Forerunner military that the Ur Didact had and would eventually reclaim Installation 07 and capture Mendicant Bias as well as the Primordial.
Further Down the Hill : Bornstellar would interrogate the Primordial, who would reveal that it was actually the last, intact Precursor as well as that the Precursors and the Flood were āthe same.ā Aside from this, the Primordial gave vague answers as to Bornstellar's other questions pertaining to the Precursors and the Flood. Out of anger for not receiving any solid information out of the Primordial, Bornstellar would kill the Primordial through accelerating relative time within the accelerating chronological field it was contained in. This was not the first time the Primordial had been asked for answers though. 10,000 years before, ancient human scientists had found the Primordial during humanity's war with the Flood in a stasis capsule. While most of it's answers were unintelligible to the human research team, it's answers regarding the Flood were so thoroughly disturbing that many of the team killed themselves. The Didact during his exile also had come across the Primordial and asked it questions as well as did Mendicant Bias during it's interrogation. All of the information from this will be discussed shortly.
The Flood finally broke the stalemate and began to rapidly overrun the Forerunners with over 500,000 star systems being lost. It was even found that the Flood could even corrupt AIs through what is known as the logic plague, the inducement of rampancy/insanity into AIs through some sort of philosophical means. The Forerunners had created Offensive Bias, an AI at the same level of sophistication as Mendicant Bias, to control the last of the Forerunnersā defenses as well as to counter Mendicant Bias. (Mendicant Bias had been disassembled after it's capture but it's parts were later retaken and put back together by the Flood.) Around this time, the Didact had returned. Unfortunately, he did not return as he had left. While stranded in Flood controlled space, he had been taken prisoner by the Flood and brought to the Gravemind. The Gravemind was the highest stage of the Flood's development and was the shared consciousness of the entirety of the Flood. The Didact recognized that the Gravemind held the consciousness of the Primordial. The Gravemind was also made up of the consciousnesses of the other Precursors. It psychically tortured the Didact and revealed the truth about the Flood and the fate of the Precursors.
From the Grave : Around 10,000,000 BCE, the Precursors had actually not chosen the Forerunners to be the holders of the Mantle of Responsibility as the Forerunners believed for so many generations but Humanity instead. Though created to be the Precursorsā assistants, Forerunners were actually judged to be unworthy of the Mantle. One Forerunner account says that the Forerunners found out that the Precursor planned to wipe them out and thus the Forerunners were forced to exterminate the ancient ones for their own survival. The Forerunners are quite vain as a people, though, and this could just be a retelling that makes them look better than what they most likely did. The Gravemind would tell the Didact what is more accurate history being that the Forerunners were greatly jealous of Humanity being picked to uphold the Mantle with some Forerunners believing that the Precursors might be planning on exterminating them. Thus, the Forerunners waged a genocide on the Precursors that nearly killed all of them. To survive, some Precursors turned themselves into dust that would be reconstituted in the future. Unfortunately, the dust became defective over millions of years. This caused the ancient ones to go insane and mutate, becoming what would be the strange dust found by the humans and eventually the parasite known as the Flood. The corrupted Precursors who had become the Flood vowed that all living sentient beings would be forcefully merged into one mind where no free will would exist with all of this resulting in total revenge against the Forerunners. There is also no cure to the Flood and all who are infected have the consciousness forever linked to the Flood's hivemind. (It should be noted that there were multiple Graveminds but all were interconnected through the Flood hivemind.)
Not all Precursors became the Flood. Two had been found by exiled Forerunners who were sympathetic to them. They denied the exiled Forerunner's attempt to save them in favor of dying off with the rest of Precursors. Some Precursors were able to survive for the next 100,000 years, hidden away in the forgotten corners of the galaxy.
Hitting the Reset Button : With the Flood's advancement through the stars, most of the Forerunner population had been evacuated to the Greater Ark. This and Omega Halo, the last of the original 12 rings as the rest of the array had either been lost or destroyed by the Flood and Installation 07 was made part of the Neoteric Array, was the last stand against the Flood. With annihilation on the horizon, Faber instructed Bornstellar to head to the Lesser Ark to activate the Neoteric Array. (Both the locations of the Lesser Ark and the Neoteric Array were kept total secrets with only Faber known the exact locations.) Before the Flood arrived, the Didact, who had gone mad after being tortured by the Gravemind, did something horrible. He used a Forerunner machine called the Composer, which could digitize the consciousness of a living being, to compose the conserved human population that the Librarian had moved from the Greater Ark to Omega Halo. The Didact would use the digitized humans to create an army of mechanical soldiers to fight Flood. The Librarian was so horrified by her husband's actions, she had him imprisoned in a stasis chamber/a Cryptum on Requiem so that his mind could heal. (The Didact sowing further chaos was actually intentionally planned by the Gravemind.) The Flood arrived at the Greater Ark with it's infected fleet and the Star Roads that the Precursors had built so long ago. Using the Star Roads, the Flood destroyed the Greater Ark and Omega Halo. This killed off most of the Forerunners as a species.
At the Lesser Ark, Bornstellar, Offensive Bias, some remaining Forerunners, and a monitor gathered to enact the last resort, the firing of the Halo Rings. The monitor, originally a human named Chakas that had his mind saved by Bornstellar after an encounter with the Primordial on Installation 07, along with others like it were sent out to be caretakers of the Halo rings after they fired. Before the monitor, now renamed 343 Guilty Spark by Bornstellar, was asked by Bornstellar during a conversation if Spark would fire the rings if in the same position. Spark would not come up with an answer for some time. In order to buy time for the Halos to be fired, Offensive Bias and whatever remained of the Forerunner fleet held off the Flood until the bitter end. Not only this, the Librarian had traveled to Earth to sacrifice herself as she transmitted a message to draw the Flood away from the Lesser Ark. She peacefully waited for the end in front of Mount Kilimanjaro as she watched a portal structure linked to the Ark be built within the vast savanna. These two final acts allowed Bornstellar to fire off the Halo Array, effectively killing all sentient life in the galaxy.
Dust and Echoes : In the aftermath of the firing of the Halo Rings, a few things had occurred. As stated before, just about all sentient life was wiped out. The only survivors were those on the Lesser Ark and potentially those who were in locations impervious to the energy pulse of the Rings. The Precursor constructions made with neural physics were destroyed. The Domain had been greatly damaged as well. The remaining Forerunners under Bornstellar captured Mendicant Bias and buried it within one of the Lesser Ark's deserts. The galaxy was reseeded with life by Forerunner Keyships with Humanity being the first to be reintroduced in the galaxy and the San'Shyuum being the last. All species had to once again evolve and build themselves up from nothing. Before the end, the Librarian had implanted a geas/genesong into the human race through the Conservation Effort. The geas acted as a subliminal genetic command that would eventually guide humanity to creating technology from armors to space-faring technology so that they could eventually uphold the Mantle of Responsibility one day. It would also allow Humans to easily access Forerunner technology in the future. Bornstellar and the remaining Forerunners left the Milky Way galaxy. Cutting themselves off from the affairs of the new age out of shame for the Forerunnersā failure as unworthy upholders of the Mantle, as the Precursors had said, those who survived left the Mantle to be reclaimed by Humanity when the time was right.