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The Diluvial War or Diluvial Conflict, descriptively known as the Forerunner-Flood war, was a massive conflict between the Forerunner Ecumene and the Flood parasite that occurred throughout the Milky Way galaxy roughly 100,000 years before the present. While records of the conflict are fragmentary, we know the war lasted for at least three centuries and ended with the Halo Event, known to the Covenant as the Great Purification or the First Ascension, which saw the firing of the Halo Array to sterilize the galaxy of sentient life in 97,445 BCE. This rid the Flood of viable hosts and ultimately starved it to death. The galaxy was subsequently repopulated by Forerunner automata in the Reintroduction stage of the Conservation Measure to preserve life, though only a fraction of the galaxy's former sentient life are suspected to have been successfully indexed and reseeded.

The best-known Forerunner individuals from this era are the Didact, the overall commander of Forerunner military forces, the Librarian, the head of the Lifeworker rate who oversaw the Conservation Measure, and the Master Builder, the mastermind behind the Halo Array. Also notable is the AI Mendicant Bias, who betrayed its creators and defected to the Flood, and Mendicant's later counterpart, Offensive Bias, who ultimately defeated the traitorous construct. Many other Forerunners are mentioned in various records, though the conflict is usually personified in the actions of the aforementioned near-mythic figures.

In the Covenant religion

In the Covenant religion, a version of the Diluvial War and the surrounding events is often recounted as the "Diluvial Narrative" or the "Last Days of Creation" — the Forerunners' epic struggle against the Flood and their subsequent ascension to godhood.