Valve has announced that the first game add-on for Left 4 Dead 2 will be available tomorrow for the Xbox 360 and PC. Xbox 360 gamers will have to cough up 560 MS Points for the new DLC while PC gamers can get their hands on it for free.

The Passing DLC brings new single player, multiplayer and co-operative gameplay to the co-operative zombie shooter, and brings the original Left 4 Dead 1 survivors down south for a meeting with the sequel’s cast. Set in a small town in rural Georgia, the DLC takes places just after the Dead Center campaign of Left 4 Dead 2 as players once again step into the role of the sequel’s survivors.

The new DLC will also introduce a series of weekly game mode events dubbed “Mutations.” According to the developer these “week-long runs of new gameplay modes range from ‘Realism Versus’ mode to ‘Chainsaw Massacre,’ which gives players chainsaws with an unlimited supply of gasoline.” Over 20 Mutations are planned, each playable on all the Left 4 Dead 2 campaigns, but the Mutations are exclusively available to those who download the DLC.

The Passing also offers a new co-op challenge mode of play, support for infected bot play in Versus, a new uncommon common zombie class known as the fallen survivor as well as a new melee weapon (the golf club) and a new firearm (the M-60).

If you haven’t yet enjoyed the zombie slaying action of Left 4 Dead 2, why not read our review or browse through our previous coverage to find out what you’ve been missing.