Set in rural Georgia, "The Passing" sees the return of the original Left 4 Dead survivors as they cross paths with the new cast. Unfortunately, it seems Bill and the rest of the old crew will be unplayable, but the idea of eight survivors fighting the horde together is cool nonetheless. It also marks the first time players actually encounter other uninfected humans -- apart from shouting through doors.
The upcoming DLC is also said to contain a new uncommon infected, as well as a new melee weapon and firearm. An exact release date and pricing haven't been set, though I'm guessing it will probably be free on PC and about $7 for Xbox 360 players.