Killing Floor (2009)

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 170 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 Ten different monster types trying to eat your face off, armed with everything from teeth and claws, to chainsaws, chain-guns and rocket-launchers 33+ 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 Windows-only SDK for the creation of new levels and mods