Blood Harvest posterThe slow and steady march (undead, you might say) to release all of Left 4 Dead 1's maps into Left 4 Dead 2 continues, with Valve offering fans a chance to unlock the beta version of Blood Harvest early. So, how does it work? Simple: To unlock Blood Harvest, PC players simply have to help test a new version of Dead Air.

Specifically, players will be awarded an Achievement for completing an entire Versus game in Dead Air. Once sixty-thousand (60,000) players unlock the Achievement, Valve will unlock Blood Harvest on the following weekday. Be warned though, it must be a complete game, as Valve notes "you can't rage quit and you need to work together."

Remember that the next time you willingly allow a Jockey to ride off on your friend.

This article was originally published on Joystiq.