environment) experiences in Halo 3: ODST, Gears of War 2, and Call of Duty: World at War manifested themselves as more straightforward horde modes where teams of four or five would hold out as long as they could on maps riddled with zombies, aliens, or monsters, developer Valve South (later re-established as Turtle Rock Studios) took things one step further.

In the late 2000s, cooperative multiplayer games that pit you and your friends against waves of AI-controlled enemies were all the rage, and Valve’s Left 4 Dead was the king of this subgenre.