Check out the official release video and PR below.

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 and 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.

In addition, “The Passing” offers a new co-operative challenge mode of play, support for infected bot play in Versus, a new “uncommon common” zombie class (the fallen survivor), melee weapon (golf club), and firearm (M-60).

Available for 560 Microsoft Points on Xbox 360 and free on the PC, “The Passing” takes place just after the Dead Center campaign of L4D2. Set in a small town in rural Georgia, players assume the role of the L4D2 Survivors as they meet with the L4D1 characters.