Forerunner-Precursor war

"I am the last of those your kind rose up against and ruthlessly destroyed."

- The Primordial to the Didact.

Ten million years before the Forerunner-Flood war, the Forerunners rose up and destroyed their creators, the race of beings known as the Precursors, in a massive war.

Overview
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 Path Kethona Forerunners' biological memory reservoir, which, in turn, recorded the memories of the early Forerunners who fought in the war. However, the Gravemind stated that the Forerunners struck against their creators completely unprovoked, enraged by the Precursors' decision to pass the Mantle on to 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.

Aftermath
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. However, over time the dust became defective and it 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.

List of appearances

 * Halo: Cryptum
 * Halo: Primordium
 * Halo: Silentium
 * Halo Mythos: A Guide to the Story of Halo