Source is a 3D game engine developed by Valve Corporation. It debuted in June 2004 with Counter-Strike: Source and shortly thereafter Half-Life 2. Other games using the engine include the physics sandbox Garry's Mod, first-person action game Dark Messiah of Might and Magic and the first-person beat-em-up Zeno Clash.
Alien Swarm Source came out 2010, Alien Swarm: Reactive Drop was released 7 years later.
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