L4D2 promises to set a new benchmark for co-operative action games and become one of 2009's marquee titles, adding melee combat to enable deeper co-operative gameplay, with items such as a chainsaw, frying pan, axe, baseball bat, and more.

Introducing the AI Director 2.0, L4D's dynamic gameplay is taken to the next level by giving the Director 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.

After debuting "The Parish" campaign (set in the New Orleans' French Quarter) at E3 and "Swamp Fever" at Comic Con San Diego, Dark Carnival is the third of the five campaigns shipping with L4D2 this fall.

Valve is also unveiling a new Boss infected monster coming in the full version of Left 4 Dead 2. Known as "The Jockey" this is the third new Boss infected from L4D2 revealed thus far (the Charger was revealed at E3, The Spitter at Comic Con). The Jockey lets you attack lone Survivors and wrestle control of their movement away from them-into fire, off ledges, or straight into the middle of a zombie horde.

In addition to the new campaign and boss monster the PAX showing of L4D2 marks the debut of the "Zombie Clown," the "uncommon common" exclusive to the Dark Carnival campaign, and a few more of the 20-plus new items coming in the full product (electric guitar, machete, and the American baseball bat being offered as a pre-sell exclusive with select retailers).

In addition to the new campaign and boss monster L4D2 also includes "Mudmen," the "uncommon common" exclusive to the Swamp Fever campaign, and a few more of the 20-plus new items coming in the full product (cricket bat, AK47, and grenade launcher).

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