"It’s not every day that a game like Alien Swarm gets released. Alien Swarm is a source mod which is based on an Unreal mod “Alien Swarm mod.” The developers of the game worked on other Valve titles like Left 4 Dead 2 and Portal 2 but were hired by Valve because of Alien Swarm mod. Which has since been developed into the official game Alien Swarm. It can take a little bit of time to wrap your head around that one.
The game is a top-down shooter that allows four players to play co-operatively as they progress through different science fiction themed levels. Throughout these levels you are faced with killing waves upon waves of aliens (which looks like the Dead Space aliens) with an arsenal of 40 high powered weapons. The game has some incredible statistics as well as an achievement system."