The DLC is going to be called "The Sacrifice".
In the DLC, its a 3-part campaign taking place before L4D2's The Passing. which it will show you how Bill died. Also giving us the choice killing off one of the survivors besides Bill. But in the further DLC (If there is more DLC coming), the survivors are gonna continue surviving maybe a new survivor would join the group... maybe.
Valve also is going to put The sacrifice in both L4D1 and L4D2. But adding an extra for L4D fans, they're going to put in No Mercy in L4D2.
There is also a 4 part digital comic about 190 pages long explaining the L4D survivors backstory in before the survivors meet.
Thats all I got for the info of the DLC. If I miss any info or I got any wrong explain. I wont rage.
BAD GRAMMER IS BAD!