This is a standalong game and NOT a mod for any other game.

Killing Floor is a Co-op Survival Horror FPS set in the devastated cities and countryside of England after a series of cloning experiments for the military goes horribly wrong. You and your friends are members of the military dropped into these locations with a simple mission: Survive long enough to cleanse the area of the failed experiments!

-Cooperative gameplay for up to six players against multiple waves of specimens
-Persistent Perks system, allowing players to convert their in-game achievements into permanent improvements to their character's skills and abilities
-Over 40 Steam Achievements, including Dignity for the dead for killing 10 enemies feeding on dead teammates' corpses and Hot Cross Fun for finishing off 25 burning enemies with a Crossbow
-Slow-motion ZEDtime to better watch those crucial and violent creature deaths, even in multiplayer
-Solo game mode for offline play
-Nine different monster types trying to eat your face off, armed with everything from teeth and claws, to chainsaws, chain-guns and rocket-launchers
-12+ weapons for the players to chose from, ranging from knives and fire-axes to pump shotguns, rifles and a flamethrower
-Equip your team with welders, medical tools and body armor to help your odds of survival
-Choose which Perks to play with to best balance out your co-op team against the horrors
-Open, non-linear play areas: choose when and where to fight — or run; weld doors closed to direct the monster horde down alternate corridors
-Fully-configurable, allowing players to change the difficulty level, number of creature waves, or even set up their own favorite waves of monsters
-Support for Steam Friends and other Steamworks features
-Includes SDK for the creation of new levels and mods