The Reapers are destructive intelligent machines who periodically awake from hibernation in the depths of space to consume all sentient life in the galaxy. They appear as giant Cthulu-esque spaceships, and have the power to control the minds of their minions. The Geth worship the Reapers as the ultimate evolution of artificial life, and worked with Saren to help bring them back to the galaxy. Most dismiss the Reapers as myth, even after Sovereign's attack on the Citadel.
Council race: no
Hated by: everybody, except the Geth
-Sovereign
Sovereign was the scout the Reapers left behind after their last harvest 50,000 years earlier, when they wiped out the Prothean civilization. He sensed that it was time for the Reapers to return, and awakened. His efforts to send the wake-up signal to the rest of the Reaper fleet was stymied by Shepard and company, with a final pitched battle around the Citadel leading to Sovereign's destruction.
Council race: no
Hated by: everybody, but especially the Quarians and humans
