The sound comes from the Zombies mode in Call of Duty, first introduced as Nazi Zombies in Call of Duty: World at War in 2008 and expanded in later Black Ops games. Round based survival made the start of each wave a ritual moment, with audio cues warning players that faster and stronger zombies were coming. Clips of the cue spread through gameplay videos, edits and soundboards, and it remains used as shorthand for a new horror round beginning today.