Flood (Halo)

A race of parasitic creatures from the original Halo trilogy. Sickly-colored and packed with tentacles, the Flood typically use small infection forms to latch onto hosts and rapidly convert them into combat forms, though a single spore can be sufficient to trigger infection. In the games' lore, they appeared seemingly from nowhere and went on to be the downfall of the Forerunners, who built the titular Halo rings to kill any creature with sufficient biomass to be preyed upon by the Flood, with only a handful of species spirited away and preserved. They are, in reality, the heavily-mutated forms of the Precursors, an even more ancient race that was thought to be extinct.