Valve will release the Dead Air map from the PC Left 4 Dead 2 DLC pack early, July 22nd to be exact...but only if twenty thousand players earn an Achievement before the end of play this Sunday.
The initiative is to try and get community help in bug-testing the titiular map, Cold Stream, for any glitches and such.
"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 explains on the L4D blog. "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."
And that community has to be twenty thousand strong; if Valve clocks that number earning the Stream Crosser Achievement (Survive a campaign of Cold Stream on any difficulty) by Sunday's end, the DLC map will release early.
For those wanting to join in, a beta version of the map is available from the L4D2 campaign list when you boot the game up.