Left 4 Dead 2 has received a heaping amount of new content in the form of the Mutation mode that provides weekly game variants (most of them good) and DLC campaign The Passing. Left 4 Dead 1 is going to join the fun too with The Sacrifice, its first DLC since Crash Course last September.
The Sacrifice will cover the events leading up to Bill's death in The Passing, and it will be played from the Left 4 Dead original cast's perspective.The Sacrifice can be played from either Left 4 Dead or Left 4 Dead 2, and if you are playing it in Left 4 Dead 2, you can play the No Mercy campaign with all the Left 4 Dead 2 weapons and enemies. All of this can be played on both PC and Mac, so Valve isn't kidding with the Mac support stuff.
Outside of the game, there will be a 150+ page comic released to detail the Left 4 Dead 1 survivor’s lives during and before the infection. In this comic, Bill will die for sure, but you can choose who dies when playing The Sacrifice. This is pretty cool that you can choose who dies, but I doubt it will have any impact on the canon of Left 4 Dead (I know that I'll change who dies, seeing as Bill is my favorite character).
