Left 4 Dead fans will no doubt be aware that Valve is hard at work on the Cold Stream downloadable content for Left 4 Dead 2. But what you might not know is that the developer has revealed an incentive for all you zombie-killing players out there.

If 20 000 players unlock the Stream Cross achievement on Steam (that is to finish the Cold Stream campaign on any difficulty level), Valve will release the Dead Air campaign from the upcoming Cold Stream Left 4 Dead 2 DLC early, on the 22nd of July.

The reasoning behind this plan is simple really, Valve are still tweaking and making gameplay changes to Cold Stream and want people to “pound on the Cold Stream maps and help [them] find any bugs.”

The Cold Stream DLC has been in beta testing for a few months now and is a collaborative project between the Left 4 Dead community and Valve.

Accoridng to a post on the Left 4 Dead blog, “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.”

“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.”