Titled The Passing, the DLC opens up new single player, multiplayer and co-operative modes in the zombie survival first-person shooter.
Priced at 560 Microsoft Points (£4.50) or free on PC, The Passing is set just after Left 4 Dead 2's Dead Centre campaign.
In a small town in rural Georgia, players will take on the role of the Left 4 Dead 2 survivors as they meet up with the original team from Left 4 Dead.
The pack also includes a new gameplay mode called Mutations, which involves a different challenge being launched every week, such as Chainsaw Massacre where all player have chainsaws with unlimited fuel.
Valve currently has 20 different Mutations planned, which will be playable on all the available Left 4 Dead 2 campaigns.
The in-game blog will let players know on Thursdays which new Mutation is coming up, while a public vote will be held every month for favourite modes to re-appear.
The Passing also offers a new co-operative challenge and a fallen survivor zombie class, along with a golf melee weapon and an M-60 firearm.