Valve has announced the first downloadable add-on for their online co-operative zombie FPS, Left 4 Dead 2. Targeting an early 2010 release, “The Passing” downloadable content will bring the survivors of the original Left 4 Dead down south to meet the survivors of the second game.

“The Passing” takes place after the “Dead Center” campaign in Left 4 Dead 2 and promises to deliver new single-player, multiplayer, and co-op gameplay. Players will meet the original game’s cast and play as the cast of Left 4 Dead 2.

Also included in the content is new arenas for Survival, Versus and Scavenge modes as well as a new co-cop challenge mode. A new “uncommon common” zombie class will be introduced as well; new weapons and firearms, too.

Look out for the DLC in early 2010.