Left 4 Dead 2’s upcoming downloadable content pack, Cold Stream, will feature a new level for which the map pack is named and revamped versions of the original Left 4 Dead maps.

The Cold Stream map is currently in open beta. Today, Valve has announced that if 20,000 people earn the “Stream Crosser” achievement for completing the map on any difficulty by Monday morning NZST, it will release the revamped Dead Air map for Left 4 Dead early.

The offering is limited to the PC version of Left 4 Dead 2.