Valve has revealed the first game add-on for its co-operative zombie thriller, Left 4 Dead 2, is in development and is aiming for a Spring 2010 release..

Dubbed "The Passing," the first game add-on for L4D2 brings the original Left 4 Dead 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.

In addition to the new co-operative campaign and associated narrative, "The Passing" will include new arenas for Survival, Versus and Scavenge mode and introduce a new co-operative challenge mode of play. The Passing will also include a new "uncommon common" zombie class, melee weapon, and firearm.