There is a realm of existence so far beyond your own you cannot even imagine it. I am beyond your comprehension. I am Sovereign.
~ Sovereign introducing itself to Shepard.

Sovereign (also known as Nazara) is the main antagonist in Mass Effect. He is a member of an ancient race of evil, sentient starships called the Reapers who want to assimilate all the other organic life in the galaxy. It is controlling Saren Arterius, and wants him to find the portal called "The Conduit" which will allow Sovereign and its fleet to launch an attack on the Citadel. It is known for its tremendously deep and intimidating voice, which was provided by Peter Jessop.

Being approximately two kilometres in size, Sovereign is not directly fought as a boss in the game, but is fought indirectly at the end where it takes direct control over Saren's corpse during the final battle, right after which it is destroyed by the Alliance fleet while trying to finish activating the Citadel relay.