Zombies, originally known as Nazi Zombies, is a game mode appearing in Treyarch games that first appeared in Call of Duty: World at War. It became highly popular upon the game's release, and returned in its sequels Call of Duty: Black Ops, Call of Duty: Black Ops II and again in Call of Duty: Black Ops III. In Zombies, players face zombies of various nationalities depending on the location of the map, including Germans, Americans, Russians and Asians. Zombies content is only considered canon in its own universe.