The company also provided details on "Mutations," described as "a series of weekly game mode events" that " range from 'Realism Versus' mode to 'Chainsaw Massacre,' which gives all players chainsaws with an unlimited supply of gasoline."

[Original - 1:00pm CST] Following up on the flood of details, Left 4 Dead 2's "The Passing" downloadable add-on will hit the Xbox 360's online Xbox Live Marketplace on Thursday, April 22, store operator Microsoft has confirmed.

Developer Valve has yet to confirm if Left 4 Dead 2 PC owners will receive the downloadable campaign, which features cameos from the crew of the original Left 4 Dead, on the same day. However, the company recently pledged that both PC and Xbox 360 would get "The Passing" this week, and the two downloadable add-ons previously released for the original Left 4 Dead hit both platforms on the same day.

"The Passing" will sell for 560 Microsoft Points / $7 on Xbox 360. Meanwhile, Valve traditionally releases such content on PC in the form of a free automatic update.