Valve, creator of best-selling game franchises (such as Half-Life and Counter-Strike) and leading technologies (such as Steam and Source), today announced Scavenge, the fourth game mode to be included in its upcoming zombie thriller, Left 4 Dead 2.

Scavenge mode challenges players to compete in round-based games of up to 8 players (Survivors vs. Infected). The Survivors must battle both the Infected and the clock as they collect gas cans to refuel a generator located in each map. Every can the Survivors pour into the generator scores their team a point and adds time to the clock.

On defense, the Infected team plays as the boss Infected (including the new Spitter, Jockey, and Charger). They must keep the Survivors from fueling the generator by attacking them, destroying the gas cans, or running out the clock. During a round, each team gets an opportunity to play both sides, with the round going to whichever team successfully delivers the most cans.

Coming exclusively to Xbox 360 and PC, L4D2 promises to set a new benchmark for co-operative action games and become one of 2009's marquee titles. Set for release on November 17, the title adds melee combat to enable deeper co-operative gameplay, with items such as a chainsaws, frying pans, axes, baseball bats, and more.

L4D2 also introduces the AI Director 2.0, adding the ability to procedurally change weather effects, world objects, and pathways in addition to tailoring the enemy population, effects, and sounds to match the players' performance. The result is a unique game session custom-fitted to provide a satisfying and uniquely challenging experience each time the game is played.

Finally, with new Survivors, boss zombies, weapons, and items, Left 4 Dead 2 offers a much larger game than the original, featuring more co-operative campaigns, more Versus campaigns, new Survival maps, and a brand new competitive mode all available at launch.