Developer Valve announced that the forthcoming Survival mode downloadable content Left 4 Dead will arrive no later than April 20. They specifically added that the DLC will debut sometime before the L4D: Critics Choice Edition re-release to retail on April 21.

The free download for PC and Xbox 360 adds the Dead Air and Death Toll campaigns to the game’s competitive Versus mode; it also adds the Survival mode.

Survival mode has players face off against hordes of enemies. Valve notes that none of its testers have lasted over nine minutes in this mode.