A brief account of the Forerunner’s plight through the eyes of Cortana.
Approximately one hundred thousand years before the war with the Covenant, there was another conflict—one which spanned the breadth of the galaxy and claimed the lives of billions. This was the legendary war between the Forerunners and the extragalactic parasite known as the Flood.
Before the Flood’s arrival, the Forerunners were an advanced civilization who dominated a vast portion of the galaxy—championing peace and security for the benefit of all living things. But when they made first contact with the parasitic threat, they failed to act quickly and decisively, and the Flood spread. Every world within the Flood's grasp fell prey to its infection—one which seemed to have no limitations.
Finally realizing that the survival of the galaxy was at stake, the Forerunners summoned every intellectual achievement they could muster, every technology at their disposal, to fight back in defense of the worlds they protected. Using immensely destructive weapons, ships of unparalleled power, and artificial intelligence constructs which outstripped anything that has been seen since, the Forerunners fought to wipe the Flood from existence.
But even with these tools, they soon realized that defeat was inevitable, and they determined that the only acceptable and plausible solution was self-destruction on a galactic scale. They created an array of seven ringworld installations which were collectively called Halo. Spreading these superstructures throughout the galaxy, the Forerunners intended to wipe out the Flood’s source of growing strength: all sentient beings, since the Flood spread by taking over such creatures.
Although this unbelievably horrific act would claim the lives of billions, the Forerunners had prepared a contingency plan: they safely stored populations of each species in order to repopulate their home worlds in the wake of Halo’s firing. This act allowed sentient life to persevere in the face of what would have been complete and utter annihilation.
Now all that remains of the Forerunners are dormant artifacts and relics strewn throughout the galaxy—a reminder of the price which was paid so that humanity and all other sentient species could survive.