The short version is, this adds a campaign which occurs after Dead Center. The L4D2 characters will meet up with the original four survivors. It will add a new co-op gameplay mode, melee weapon, firearm, and uncommon infected.

No word on price for the PC, but due to Microsoft’s policies, you can expect there to be some cost for the Xbox 360.

Valve’s official press release follows:

“The Passing” Brings L4D1 Survivors Down South

December 14, 2009 – Valve, creators of best-selling game franchises (such as Half-Life, Portal, Team Fortress, and Counter-Strike) and leading technologies (such as Steam and Source), today announced the first game add-on 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.

With the AI Director 2.0, L4D’s dynamic gameplay is taken to the next level by giving the Director the ability to procedurally change weather effects, world objects, and pathways in addition to tailoring the enemy population, effects, and sounds to match the players’ performance. The result is a unique game session custom fitted to provide a satisfying and uniquely challenging experience each time the game is played.

Finally, with new Survivors, boss zombies, weapons, and items, Left 4 Dead 2 offers a much larger game than the original, featuring more co-operative campaigns, more Versus campaigns, new Survival maps, and the new competitive game mode, Scavenge.

Left 4 Dead 2: The Passing is targeted for release in early spring for the PC and Xbox 360. Pricing will be announced closer to launch.

For more information, please visit www.l4d.com