Valve, creators of best-selling game franchises (such as Half-Life, Portal, Team Fortress, and Counter-Strike) and leading technologies (such as Steam and Source), today announced the first game add-on for its co-operative zombie thriller, Left 4 Dead 2 The Passing, is due for release Thursday, April 22 exclusively for the Xbox 360® video game and entertainment system and the PC.The first game add-on for Left 4 Dead 2 (L4D2) brings the original Left 4 Dead (L4D1) Survivors down south for a meeting with the L4D2 cast, while delivering new single-player, multiplayer and co-operative gameplay for the PC and Xbox 360.

The Passing also introduces a series of weekly game mode events dubbed “Mutations.” These week-long runs of new gameplay modes range from “Realism Versus” mode to “Chainsaw Massacre,” which gives all players chainsaws with an unlimited supply of gasoline. There are over 20 Mutations planned, each playable on all L4D2 Campaigns. Mutations are exclusively available to owners of “The Passing” DLC.