The first downloadable add-on for zombie shooter Left 4 Dead 2 is slated to hit PC and Xbox 360 in the "early spring," developer Valve announced today.

A crossover of undead proportions, "The Passing" sees the original Left 4 Dead survivors meet up with those of Left 4 Dead 2, with players taking control of the L4D2 crew.

The download will include a new campaign, set in a rural Georgia town after the events of L4D2's Dead Center campaign, along with a new firearm, a new melee weapon, a new type of "uncommon common" foe, and new arenas for Survival, Versus and Scavenge plus "a new co-operative challenge mode of play."

Valve noted that "pricing will be announced closer to launch." The company is known for its extensive free post-release support on PC--see Team Fortress 2--but has lamented that Microsoft forces it to charge for some Xbox 360 add-ons.

"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 Valve marketing VP Doug Lombardi. "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."