Valve will release Dead Air from the upcoming Cold Stream Left 4 Dead 2 DLC on 22nd July if 20,000 people can earn the Stream Crosser achievement by the end of this Sunday.
To get the Stream Crosser achievement, you need to survive a campaign of Cold Stream on any difficulty.
"Normally on an internal project we would have the whole company take a part of each day and do nothing but play the upcoming game or DLC and submit bugs," Valve wrote on the L4D blog.
"To add to the fun we often give out prizes for participating. Since Cold Stream is not just being developed internally but also with the community - we want the community to help pound on the campaign and look for bugs."
The Cold Stream DLC, which has been in beta testing for a few months now on PC and Mac, is a collaborative project between Valve and the Left 4 Dead community.
It started life as a community campaign created by modder Matthew Lourdelet before catching the eye of the folks at Valve who then decided to jump in, buff it up and give it a full release.
Tom gave Left 4 Dead 2 a 9/10 for Eurogamer in November 2009. "It's an amazing volume of new modes and features for a game that once kept things simple, but it shouldn't come as too much of a surprise to see them," he wrote.
"Whereas once we treated Left 4 Dead as a stopgap between Half-Lifes, this is no longer a weird little side project with modest expectations, and Valve is confident enough to play around with it, safe in the knowledge that you can trust your players. Left 4 Dead proved it. And whereas that game had a personality, this one is overflowing with it."