WARNING: The following briefing contains spoilers for several of the Halo books. Proceed with caution.
Around 100,000 years before the events of the first Halo game, the Forerunners were a race of technologically superior beings that conquered and controlled the entire galaxy. They had mastered nearly every aspect of science and technology, having the ability to move planets, slow time, and trap stars.
Another key factor in Forerunner history was their relationship with humans. At a time, Forerunners and the combined empires of humans and Prophets stood as equals in the galaxy. In fact, the Forerunners were almost unable to stop human expansion, until a calamitous infection invaded the human worlds.Living for thousands of years at a time, the Forerunner would metamorphose into a class in their rigid caste system. In order of lowest to highest ranking, there were Lifeworkers, who indexed the genomes of every species in the galaxy, Miners, Warrior-Servants, who fought the Forerunner wars and were responsible for bringing every other sentient species in the galaxy to their knees, and Builders, whose great works include the massive ring worlds, called Halos, and Requiem, the Dyson Sphere-like setting of Halo 4.
The Flood War:
Millions of years ago, the Forerunners had risen to power by rebelling against the Precursors, the pinnacle civilization in the history of the galaxy. As a measure of last resort and final retribution, the Precursors sent a ship on a round trip out of the galaxy. Eons later, the ship returned and landed on a human world, carrying with it hundreds of canisters of fine powder. This powder jumped from species to species, until it started to infect humans and Prophets. The infection was a parasite that commandeered the host, gaining its knowledge and functions.
The Flood, as the parasite came to be called, was on the verge of wiping out prehistoric humanity, which was also engaged in a prolonged, widespread war with the Forerunners. Facing extinction, the humans and Prophets developed a method of vanquishing the Flood. They willingly changed the genetic code of one third of their population and placed them in the path of the Flood. When infected, these new Flood would turn on each other and destroy themselves. One ship, filled with unaltered Flood, escaped the galaxy.
Humanity was saved, but the brash tactic had taken its toll. After enduring the Flood and sacrificing a third of its people, humanity was unable to defy the might of the Forerunners. A special group of Warrior-Servants, called Prometheans, were dispatched to end the war against the humans. The Human and Prophet Empires were demolished, and the remnants of their populations were relegated to technologically devoid planets, where they would stay for ten thousand years.
Much like the canister-containing ship before it, the lone Flood vessel returned to the galaxy, this time landing in Forerunner-controlled space. The Forerunners had mostly forgotten about humanity’s brush with the Flood, leading them to treat the Flood casually as a disease, rather than a galactic parasite. The Flood, immediately able to command the advanced Forerunner technologies wielded by those they infected, was able to quickly gain the upper-hand against them.
To make matters worse, the Forerunners were in a period of disarmament, believing that there was nothing left in the galaxy that could oppose them. Instead of having standing armies and navies filled with Warrior-Servants, the Builders constructed the Halos, an array of twelve rings that, when fired in tandem, would annihilate anything with a neural system in the galaxy. The Forerunners created the most powerful, advanced AI in existence to control the Halos and all other Forerunner defenses. They called it Mendicant Bias.
The Primordial, a creature created by the Precursors and left in slumber for eons, was unleashed by Mendicant Bias test-firing a Halo on an ancient Precursor world. The creature was subsequently captured by the AI and transported for interrogation to one of the Halo rings. Over a period of decades, the creature slowly corrupted Mendicant Bias and convinced it to rebel against its creators. It commandeered five of the Halo rings to destroy the Forerunner capital.
The other seven Halos were saved from Mendicant Bias and transported to the Ark, which was a Halo-like structure resting well outside of the Milky Way that served several purposes. First, it held the indexed genomes of every species in the galaxy and had the capability to repopulate it. Second, it could repair damaged Halos. Lastly, it was the only way for the entire Halo array to fire at once.
Mendicant Bias and its fleet of millions of Flood-infected Forerunner ships rapidly eviscerated the remaining Forerunner forces in the galaxy. Hearing of the Ark and the remaining survivors’ plan to activate the Halos, Mendicant Bias gathered his forces and mounted an all-out attack on the Ark. The Forerunners had created another powerful AI, called Offensive Bias, for the sole purpose of stalling Mendicant Bias’ fleet long enough for the Halo array to wipe the galaxy clean of life and Flood.
Offensive Bias succeeded and the Forerunners, in their most desperate hour, fired the seven remaining Halos and obliterated all biological mass in and around the galaxy. Without the Flood at its disposal, Mendicant Bias’ fleet was demolished, and the AI was split into several pieces and scattered across the Milky Way.
Soon, the robotic Forerunner Sentinels went about repopulating the galaxy to its previous state. The Forerunner, however, were not included in this scheme. They had deemed themselves unworthy to rule the galaxy and, instead, passed on the mantle to humanity, who they labeled Reclaimers. Any remaining Forerunners and Flood Graveminds were stored on the Halo, Installation 07. Several samples of the Flood were kept in various labs around the galaxy for study.