Flood
The Flood (originally called the shaping sickness) is a parasite which is driven by a desire to infect, kill or consume all sentient life it encounters. The Flood first appears in Halo: Combat Evolved, when the Covenant release some captured specimens from stasis on Installation 04; the parasite spreads and threatens to escape the ring, until the Master Chief destabilizes the installation with a massive explosion coming from the Pillar of Autumn.
Read more about this topic: Forerunners (Halo)