Valve's latest batch of Left 4 Dead DLC has been announced. It's coming to both Left 4 Dead and Left 4 Dead 2 on October 5th and, on the PC, it's completely free. It's also adding a new way of playing online: "sacrificial gameplay".
Both versions of the game will receive a new campaign called The Sacrifice, set just before the events of The Passing, which will centre around the original Left 4 Dead survivors as they fight their way south. The campaign will include a whole new type of finale which will include 'Sacrificial Gameplay', where players "get to decide who will give their life so the others may live," which sounds as though it might ruin a few friendships.
The Left 4 Dead 2 version of The Sacrifice will also have you playing as the original survivors from Left 4 Dead 1, but will also include all of the extra weapons and special infected that Left 4 Dead 2 players are used to, so you'll be able to play as Zoe and chop up Chargers with a Chainsaw. As an added bonus, the whole No Mercy campaign from the original Left 4 Dead is being ported to the sequel, and will include additional support for L4D2's Scavenge mode.
The Left 4 Dead comic , also called The Sacrifice, is currently being released, one part at a time. Perhaps it'll offer some clues as to where we'll be fighting the infected in the new campaign.