Valve has promised to release DLC ahead of schedule if enough players complete its latest Left 4 Dead 2 challenge.
The Left 4 Dead blog asks players to survive a single campaign in Cold Stream, a map currently in beta, on any difficulty setting.
Successful players will score the Stream Crosser achievement, and if 20,000 do so by Sunday, Valve will release Dead Air, part of the upcoming Cold Stream content pack, on July 22.
Cold Stream is a community campaign set to receive official release as DLC. Left 4 Dead 2 is playable on PC, Mac, and Xbox 360, and is on sale for $4.99 on Steam.