97,445 BCE
From Halopedia, the Halo wiki
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Template:Ratings The date 100,000 B.C.E. is the approximate time when The Forerunners fired the Halos[1], what the Covenant refer to as The Great Journey. The precise activation date was the year 97,448 B.C.E., and the activation was supposed to end the Forerunner-Flood War. The activation killed all sentient life within three radii of the galactic center, including, it is generally believed, the Forerunners themselves. The date, about 100,000 years ago, is around the time our Homo Sapien ancestors began migrating out of Africa. The date is also when the Halo 3: The Cradle of Life comics take place.
Mendicant Bias
The Forerunner AI known as Mendicant Bias was designed to battle the Flood but he was seduced by Gravemind and descended into rampancy. He allowed the Flood into the Maginot Sphere, which gave them the opportunity to attack the Forerunner's inner colonies. This forced the Forerunners to fire the rings and effectively commit "mass suicide". He later repented to Spartan-117 (Master Chief) through the terminals on the levels, promising to atone for his past actions by helping the Master Chief fire Halo and help him escape (how he does this is unknown).
