Valve's revealed an incentive for Left 4 Dead 2 players: if 20,000 of you unlock the Stream Crosser achievement on Steam (finish the Cold Stream campaign on any difficulty level) this weekend, it will release the Dead Air campaign from the first Left 4 Dead early, on July 22. Sorry, no word on remaining L4D campaigns Death Toll and Blood Harvest -- they're still slated to be included in the final release of Cold Stream, which is currently in beta.

Of course, because Valve is a company full of diabolical geniuses, guess what's on sale right now? Left 4 Dead 2 is 75% off today only, bringing it down to $4.99.

This article was originally published on Joystiq.

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