Originally released in 2009, first person co-op survival horror game 'Killing Floor' is having a Twisted Christmas event from December 14th 2010 - January 4th 2011. The event consists of a new map - Santa's Evil Lair - and they've also doubled the number of achievements available to almost 130, including 13 Christmas ones. By completing all 13, you can also unlock the playable character, Baddest Santa.
Just to make it extra festively bizarre, they've changed all the enemy characters into not-so-jolly Christmas-themed attackers. Gorefasts, become Gingerfast, Fleshpounds become Nutpound (complete with Nutcracker Suite theme music), Crawlers become Rudolph the Red-Nosed Crawlers and there's Mrs. Claws, Jack Frost, Elves and Santa's Little Helper amongst other holiday horrors too!
Killing Floor Twisted Christmas
Video after the cut below: