The free event includes a new level, Santa's Evil Lair, along with all of the creatures in the game replaced with holiday-themed versions of themselves (they go there with the Nutpound). Plus there will be 13 new Christmas achievements added as well as a new difficulty level, Hell on Earth. If you managed to get 10 of them you can get a new playable character, Baddest Santa. Finally there's a Team Fortress 2 crossover. Anyone who owns both games can get the red and blue Pyro character to use in Killing Floor.
In addition to the free Twisted Christmas event, a new paid DLC for the game will be released next week. London's Finest will add four new playable characters with a UK twist to the game. Pricing was not announced. You can check out a video of what you can expect from Twisted Christmas after the jump:
Gallery: Killing Floor