On April 20, 2017, an enhanced version called Alien Swarm: Reactive Drop, developed by a group of community volunteers, was released as a separate standalone game, containing numerous additional features like new co-op campaigns, a single-player mode with bots, PvP modes, support for up to eight players, Steam Workshop support, more achievements, and an expanded arsenal of weapons.

[3] Both games follow the same premise, in which each player takes the role of a space marine as part of a squad that is fighting off swarms of hostile alien parasites while attempting to accomplish various objectives.

[4] Players can choose from 40 different pieces of equipment, ranging from weapons like assault rifles to grenade launchers and offhand items like mines and healing beacons.

[8] As they advance through the level, they will find certain key points blocking the progress, which are cleared through a variety of methods, from welding to shooting to hacking.

Marines deployed by the Bloodhound dropship arrive to search for survivors and, if need be, to destroy the colony to prevent the aliens from spreading.

[14] An Alien Swarm software development kit (SDK) including buildable source code was released alongside the game.

This allows total conversion mods, which do not rely on content from other Valve games, to be free to all as well - a significant business decision that echoes the strategy of the Unreal Development Kit.