Sovereign

Sovereign is a gigantic starship of almost unimaginable power and sophistication. Commander Shepard originally believed Sovereign was a Reaper vessel that Saren Arterius found and somehow reactivated, but discovers Sovereign is in fact an actual Reaper, a fully sentient and extremely powerful AI, which is why the geth obey its commands. They see Sovereign as the pinnacle of AI evolution and worship it as a god. However, Sovereign regards the geth as no more than tools to aid in the return of the Reapers and is even insulted by the worship the geth "throw" at it, and Saren as simply a useful slave. A vanguard of the Reapers left behind to instigate their eventual return from dark space, Sovereign has spent the last 50,000 years in a state of near-constant hibernation, waking periodically to assess the evolution of organic races. It was the duty of the starship to bring about the next revolution of the cycle, and serve as the harbinger for the return of its kind by sending a signal to the keepers on the Citadel. This would make the keepers open the Citadel relay, a mass relay that leads to dark space, and usher the Reapers through.

Sovereign is also able to control organic life through the process of indoctrination. A subtle field or signal surrounds the Reaper, gradually affecting the organic's mind until they are little more than a shell of their former self, bent on doing Sovereign's will. Sovereign has nothing but contempt for any lesser beings, seeing them as fleeting and temporary, whereas the Reapers are eternal. It also seemed angered by the attempts of organic life to defend their claim to the Citadel, claiming "this station is mine."

The Invasion of Coruscant
TBW.