Valve has announced the first game add-on for its co-operative zombie thriller, Left 4 Dead 2 The Passing, is due for release Thursday, April 22nd exclusively for the Xbox 360 for 560 MS Points and the PC for free.

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).