First thing you got to do is finish The Passing. It is based on the story of the 4 survivors from the first Left 4 Dead game. Namely, Zoey, Bill, Louis and Francis. The Sacrifice is set to come out as a game and will be more exciting by giving players a chance to make their own ending.
The Sacrifice is available as a comic. From the L4D Blog, they presented the new line up for The Sacrifice release: For Left 4 Dead 2, gamers receive “The Sacrifice” campaign and L4D1’s “No Mercy” both playable with the L4D1 Survivors. This version also offers 5 new achievements. In the L4D2 version of “The Sacrifice” each campaign is playable in Campaign, Versus, Survival, and Scavenge modes, and both "The Sacrifice" and L4D2 version of No Mercy feature the Left 4 Dead 2 Special Infected, items, and weapons. Yes, Zoey with a chainsaw!-source-
Related to The Passing, I remember that the first batch of survivors were lacking Bill, the Vietnam Veteran. It brought out so much curiosity on what happened to Bill. The Sacrifice will explain it all. Can't wait!