Valve, today announced the first DLC for its co-operative zombie thriller, Left 4 Dead 2 (L4D2), is in development and targeted for release in early 2010.

Dubbed “The Passing,” the first game add-on for 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.

Targeted for release in early spring, “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.

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.

“The Passing will become the most important campaign in the Left 4 Dead story, as all the Survivors are being called together in one campaign,” said Doug Lombardi, VP of marketing at Valve. “It will also be a huge offering of new gameplay content, with something new for every game mode plus a new uncommon common and weaponry.”

Exclusively available for the Xbox 360 and PC, L4D2 sets a new benchmark for co-operative action games. The title adds melee combat to enable deeper co-operative gameplay, with items such as a chainsaws, frying pans, axes, baseball bats, and more.

We’ll have more information as it becomes available, and stay tuned to 360Sync for all Left 4 Dead 2 news!