UPDATED: 3:50 PM PST - Valve has provided some clarification about those “Mutations” referenced in the achievements list for “The Passing.” Turns out these mutations are week-long sessions based on the game's new modes such as “Realism Versus” and “Chainsaw Massacre,” the latter of which sounds particularly gnarly as each player gets a chainsaw and unlimited gasoline.

All of the 20 planned mutations can be played in each of the LFD2 campaigns, but they will be only be available to those who pick up “The Passing.” And yes, the DLC will be free for PC, and it will be released on April 22 as well.

Original story below:

Heads up, Left 4 Dead fans. “The Passing” downloadable content hits Xbox Live on April 22 (that’s this Thursday!). The highly anticipated meet up between the casts of Left 4 Dead and Left 4 Dead 2 costs 560 Microsoft Points ($7).

In preparation for the big release, the official Left 4 Dead blog has posted a list of achievements for the upcoming DLC. Most are your standard “kill this many infected” variety, but some border Spoilerville, so if you want to go into the game completely clean, stop reading now.

While the revelation that the DLC will introduce footlockers is certainly a juicy tidbit, the most noteworthy achievement is Mutant Overlord, which mentions a previously unannounced element, mutations. To unlock the achievement, you must “Play 6 Mutations.” What does that mean? I have no idea, but the answer is supposed to be coming tomorrow, so check back then.

Source: Major Nelson. Left 4 Dead blog

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