Valve has confirmed today that the free Left 4 Dead DLC would be released on both the Xbox 360 and PC on April 21st.
The pack will contain two new versus campaigns based on the other two existing campaigns, Death Toll and Dead Air. Also found is a brand new four player co-op mode called Survival, which plays a bit like Gears of War 2's Horde mode.
For more information check out our previous article on the survival mode.
Source: Destructoid.com