Today Valve announced that The Passing DLC forwill arrive on April 22nd for the Xbox 360 and PC. They also nailed down most of the specifics of the DLC.There's a lot more to the Passing than just the new campaign . It also adds weekly game events called "Mutations." A Mutation is a new gameplay mode that will be playable for a week. There are twenty Mutations planned and while they'll be playable on allmaps, they're only available if you grab the DLC. One example of a Mutation mode is "Chainsaw Massacre," in which all players have chainsaws with unlimited gasoline."In addition, "The Passing" offers a new co-operative challenge mode of play, support for infected bot play in Versus, a new 'uncommon common' zombie class (the fallen survivor), melee weapon (golf club), and firearm (M-60)," says Valve. Ten new Achievements are also up for grabs. The full list can be found at the Left 4 Dead Blog "The Passing" will be free for the PC. Xbox 360 owners will have to shell out 560 MS Points ($7) for it.