Flood

The Flood is a mythical race who is supposedly responsible for the destruction of the Precursor civilisation. Information regarding their physiology, society, and culture are unknown.

Overview
If Death truly does exist, then the Flood is his dreaded scythe. The Book of Sorrows is the only known piece in the known universe that references the near-mythical species. Their numbers are legion, and wherever they tread, a billion planets would cry out in fear. From the few bits of information the scholars of the Imperium Libraricus have been able to decode, the Precursors fought against the Flood in a conflict known as the "War in Heaven". Precursor weapons slew countless Flood; but where one fall, a thousand more would take its place. The Precursors tried hundreds of solutions against the Flood but to no avail. What happened afterwards remain a mystery, but it is believed that the Precursors were able to develop a weapon that ultimately wiped out the Flood—and themselves.