One of my all time favourite enemies in any game series, has to be The Flood from the Halo series.I can still remember the first time encountering them in the first game, many many years ago when I was a kid.That mission, and their reveal was so cool. A swamp area, misty and dark, little blips on the map appearing occasionally, only to descend back into the fog. Ominous music playing, finding nobody, no Humans, no Covenant; nothing.