Collectors

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.

Biography
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.