The Collectors are the main antagonists of Mass Effect 2. A mysterious insectoid race hailing from beyond the Omega 4 Relay, they are responsible for abducting entire human colonies in the Terminus Systems, though the purpose for doing so is unknown.
The Collectors hail from beyond the Omega 4 Relay, and are the only species to be able to move freely back and forth through it: no ship that has passed through this Mass Relay have never returned.
They occasionally enter the Terminus Systems and trade their technology in exchange for living beings in odd numbers or varieties (e.g. tow dozen left handed Salarians, sixteen sets of Batarian twins etc.).
Once the deal is complete they immediately retreat back into the Omega 4 Relay as quickly as they arrived. It is revealed in Mass Effect 2 that the Collectors are genetically modified Protheans, the race that fell victim to the Reapers in the previous cycle. They have been re-purposed by Harbinger to collect humans for harvest, with the ultimate goal being the creation of a Human-Reaper. After Shepard assaults the Collector Base and destroys the incomplete Human Reaper, he prepares to destroy the base, but the Illusive Man suggests that Shepard instead use a radiation pulse to wipe out all the collectors on board while leaving the technology intact for Cerberus to salvage and use against the Reapers. Whatever choice Shepard makes, Harbinger realises that the Collectors have failed in their mission and abandons them to their fate.
The Collecotrs return as DLC for Mass Effect 3's Multiplayer mode. A large amount of them had joined with the Reapers in hiding in Dark Space, and rjoin them in invading the galaxy in 2186. However, the Leviathan's manage to sever the Reaper's control over Collector's on the battlefield; while most of them die as a result, a small few regain their sentience and join the rest of the galaxy in the war, hoping the avenge the Prothean race.