Killing Floor

Killing Floor is a co-op horror FPS from the makers of Red Orchestra: Ostfront 41-45. It originally started off as a free co-op multiplayer mod for Unreal Tournament 2004. It was later turned into a standalone game that was released on Steam in 2009.

Your team members are deployed in London populated by hordes of failed experimental clones that are known as specimens and are often mistaken for zombies. Each game is split into multiple 'waves' of harder and harder specimens, with the final wave ending in a boss battle with a deadly specimen called the 'Patriarch'. You will need to make use of high powered weapons including rifles, combat shotguns, axes, chainsaws, rocket launchers, nail-gun, pistols, grenades and more. There are additionally 'Perks' that each player can choose from what perk they desire to level up, which will in turn improved abilities such as more ammo capacity, faster reload times, etc. This depends on what perk you have chosen to which abilities you will get.