Reduction is an addon for Valve's game, Alien Swarm - a 4-player co-operative game that is free to download and play (it's not "free-to-play" either - it's totally free).
Reduction now is a preview campaign, consisting of 4 linked fully-playable multiplayer levels: Mission's 1, 3, 4 and 5. The final, full campaign will have 12 missions when completed.
To download and play Reduction do the following:
1.) Get Alien Swarm here... This is the game that runs Reduction. I recommend you play through the included campaign (Jacob's Landing), to get a sense of how Alien Swarm plays, before you play the Reduction campaign.
2.) Get Reduction here (click on one of the links below)...Version 2.1 (30 Sept 2012)
3.) Unzip the Reduction file, and then place this unzipped reduction.vpk file into this folder on your PC computer: C:\Program Files\Steam\steamapps\common\alien swarm\swarm\addons.
4.) Restart Alien Swarm. Inside Alien Swarm go to: Options > Add-ons. Scroll down to the make sure that Reduction is checked / enabled.
5.) MAKE SURE YOUR GAME INSTRUCTOR IS TURNED ON! Otherwise you will miss the story! And you won't see the hints and instructions that guide you through the levels. These are all captions. Inside Alien Swarm go to Options > Multiplayer > Game Instructor: Set to Enabled. (In the earlier builds some players complained about getting lost. This was because they had their Game Instructor turned off and didn't get the hints. It's true that the Instructor spams help messages when first turned on - but these eventually stop coming.)
6.) Finding A Game Session: Inside Alien Swarm you can join or create Public Games or peruse the Dedicated Server Browser. Invite your Steam friends to join in Alien Swarm > Reduction. They will need the reduction.vpk installed as well.
7.) About "Onslaught Mode": I make no guarantees over how well Reduction works when playing in Onslaught Mode, since level designers have limited control over when and where onslaughts spawn.
8.) Post Your General Feedback here (gameplay, general impression, etc)...
9.) Post Your Technical Feedback here (any glitches, crashes, lag, falling outside the world, etc)...Note that Reduction is currently in beta (V1.x). As new versions come out, people will need to re-download the reduction.vpk to keep it to the most current version. I encourage your feedback as well.
Version 1.9 (3 September 2012)
Version 1.8 (5 August 2012)
Version 1.7 (2 August 2012)
Version 1.6 (28 June 2012)
Version 1.5 (17 June 2012)