From Halopedia, the Halo wiki
Most details and even the causes of this conflict remain obscure or contradictory, as any records of it had been completely lost and/or intentionally suppressed by the Forerunners over the subsequent millions of years. According to some Forerunners who later learned of the event, the Precursors had decided to erase the Forerunners from existence, as they had done with many of their other creations. The Librarian learned this information from the Sedaaro Forerunners' biological memory reservoir, which, in turn, recorded the memories of the early Forerunners who fought in the war.
However, the Gravemind claimed that the Forerunners struck against their creators completely unprovoked, enraged by the Precursors' decision to pass the Mantle of Responsibility on to prehistoric humanity instead of the Forerunners. According to the Gravemind, the Precursors did not defend themselves and simply marveled at the sheer violence the Forerunners were capable of.
After driving the Precursors away from the Milky Way galaxy, the Forerunners—under the leadership of the then-dominant Warrior rate—chased them down to the satellite galaxy of Path Kethona, where they finished their work, driving the Precursors to near-extinction. However, the Forerunners who traveled to Path Kethona never returned to the Milky Way, as they were unable to bear the burden of what they had done. Those who defied their commanders and invoked the Mantle were either executed or exiled on a barren planet and would later remain as the only Forerunner survivors of the fleet that conducted the extermination campaign at Path Kethona.
Another account made by the Curator between 2554 and 2558 with information it learned from the Domain gives a version of the events that both match up with and differ from those of the former two accounts. According to this account, the Precursors seeded life in the Milky Way before settling in Path Kethona, leaving behind only remnants of their existence in the Milky Way. These remnants would later be discovered by the Forerunners who would also learn of the Mantle and lay claim to it as they were the most technologically advanced species in the galaxy thus believed it should fall to them to uphold it. However such power made the Forerunners arrogant and prideful, which was observed by the Precursors. The Precursors did not approve of the Forerunners' brash claim to the Mantle and decided to wipe out the Forerunners and give the Mantle to another species they deemed worthy. The Forerunners discovered this when they sent an expedition to Path Kethona in search of the Precursors, ultimately encountering them. The Forerunner leadership was determined to avoid this fate and struck a preemptive strike on the Precursors, launching the largest known military campaign in the Milky Way's history at the time. They proceeded to near-exterminate their creators with only a fraction of their forces returning and then kept the conflict and its outcome a secret throughout the majority of Forerunner history.
Though all Precursors were believed to have been eradicated, the Primordial, who originally claimed to be the last Precursor, later stated that a number of Precursors survived by escaping beyond the Forerunners' reach. A few Precursors did indeed manage to survive the Forerunners' extermination efforts. They either went into suspended animation or became dust that would regenerate into their past forms when the time was right. However, time rendered the dust defective, and it only created sickness and disease. With this new form, (the earliest stage of the Flood), and embittered by the conflict with the Forerunners, the Precursors vowed that none of their creations would rise against them again.
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