Alien Swarm: Reactive Drop extends Alien Swarm, bringing more of everything: maps, aliens, game modes, guns. New co-operative campaigns Challenges: Modifications of the game, just like Mutations in Left 4 Dead 2 PvP: Deathmatch, Gun Game, Instagib, and Team Deathmatch Singleplayer: Play with improved bots on all our official maps New Aliens: HL2 antlion guards and more New Weapons: Desert …