"'The Sacrifice' is the prequel to 'The Passing,' and takes place from the L4D Survivors' perspective as they make their way South," reads the press release. "In addition to advancing the story, 'The Sacrifice' introduces a new style finale featuring "Sacrificial Gameplay" where players get to decide who will give their life so the others may live."
In L4D, 'The Sacrifice' will include the campaign and maps playable in Versus and Survival. In L4D2, the DLC will yield the same campaign, which is played using the L4D characters, and the first game's 'No Mercy' campaign as an added bonus. Both campaigns will include L4D2 weapons and Special Infected. The maps will be playable in Versus, Survival, and Scavenge modes, as well.
PC Gamer, which correctly placed the DLC's release on October 5, also reported that 'The Sacrifice' would launch alongside the Mac version of Left 4 Dead 2, but Valve made no mention of this in its release. Shacknews has asked Valve for more information on a Mac launch for L4D2 or L4D.
AGH! Don't sneak up on me like that Valve! I'm not used to it! Sounds great though. I haven't played LFD2 in a while and I would love to return with some new content available.
That's seriously the first time I've seen someone use LFD instead of L4D.