Forerunner-Flood War (Earth-2042)

"A hundred thousand years ago, a great civilization existed in our galaxy. Like all great civilizations, they faced a sudden and dire turn of events. A threat to their primacy from outside. Something they never expected, never prepared for... a deeply alien threat they called simply the Flood."

- Cortana

The Forerunner-Flood War was a pivotal galactic conflict that occurred between the Forerunners and the Flood parasite. It was started on Seaward around 97,745 BBY and lasted approximately three centuries. The Forerunners concluded the war in 97,445 BBY with the creation and activation of the Halo Array, seven superweapons that purged the galaxy of all sentient life in order to stop the Flood. The conflict ended the Forerunners' eons-long rule in the galaxy; most of their Ecumene having fallen to the Flood, the few remaining members of the species went into exile after completing the repopulation of the galaxy's sentient species from specimens preserved at the Ark.

Source

 * https://www.halopedia.org/Forerunner-Flood_war