Valve has announced that Left 4 Dead 2 downloadable add-on The Passing will be released for PC and Xbox 360 this Thursday, 22nd April.
The Passing is free for PC owners but costs 560 Microsoft Points (£4.76 / 6.72) on Xbox Live Marketplace.
The developer also revealed that The Passing will be supported by weekly game mode "mutations".
Mutations will include things like "Realism Versus" and "Chainsaw Massacre", the latter providing all players with unlimited-fuel chainsaws.
Over 20 mutations are planned and will be playable across all the Left 4 Dead 2 campaigns, not just The Passing.
However, you will need to have downloaded The Passing in order to get involved with the mutations.
In the main campaign, which takes place just after the Dead Center episode, the L4D2 survivors visit a town in Georgia and encounter the survivors from the original game. (There are some spoilery story details from Valve elsewhere on the site.)
Players will also get to try out a new co-op challenge mode, play against infected bots in Versus, and take out a new uncommon common zombie (the fallen survivor) with new tools like a golf club and an M-60.
Valve has previously said that The Passing will be followed by a downloadable add-on for the original Left 4 Dead that shows events leading up to the crossover between bands of survivors.
Check out our Left 4 Dead 2 review elsewhere on Eurogamer.