The Sacrifice

One of the newest features added onto the Left 4 Dead games is the downloadable content (360 & PC) for the campaign entitled "The Sacrifice."  There was a lot of buildup and anticipation for the latest L4D DLC, a lot of which was added throughout the official website and other L4D blogs & sites.  There was even a comic, split into four parts, put out onto the official website (which was quite entertaining from what I read).  If you read the entire comic, you can find out exactly what happens and, through that knowledge, you are able to choose your own ending once playing the actual game.  This portion of L4D is supposed to be a prequel to "The Passing" and takes place through the eyes of the survivors as they travel to the South.  When you download the game, you get full access to the maps playable in campaign, versus, and survival.  If you get this DLC through L4D 2 then you can play both "The Sacrifice" campaign and L4D 1's "No Mercy" (both of which are playable with L4D 1's characters).  Both of them also feature L4D 2's Special Infected, items, and weapons.  The trailer can be viewed here