Back in 2010 Valve surprise-released Alien Swarm, a tense co-op shooter born from the DNA of the Alien films. It's a criminally underrated game that sees four players nervously exploring bases absolutely stuffed with different sorts of violently aggressive aliens.
Every mission is spotted with moments of action where swarms of aliens pour out of the walls to surround you. An undisciplined team will be picked off by the enemies and friendly fire, making for a thrilling game. Six years on, it's getting a free sequel.
Alien Swarm: Reactive Drop aims to expand the game with new campaigns, weapons, and enemies. The main change is that it is adding Steam Workshop support, making it easier to snap on mods to the game that change how it plays:
Here is the planned feature list listed on the game's store page:
Tactical, 4 player or 8 player co-op action game with a top-down perspective
Steam Workshop support for community maps and challenges
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
Single-player: Play with improved bots on all our official maps
New Aliens: HL2 antlion guards and more
New Weapons: Desert Eagle, Devastator, Combat Rifle and more
Over 100 Steam achievements
Now the original Alien Swarm was a mod in the works for the Unreal Engine. Valve hired the team before it was released, and in between working on Left 4 Dead and Dota 2 that original mod team ported the game to the Source Engine to release it as Alien Swarm. Part of the release was that a Source Engine SDK was released alongside the game, making it possible for modders to alter the game. Reactive Drop looks to be a team of modders who have built off a significant new branch to the original game. This isn't a new game from the original Alien Swarm team.
The team hopes to have Reactive Drop finished and released in early 2017.