Sovereign is the Vanguard of the Reaper fleet, who also serves as the flagship for Saren. Like all Reapers, Sovereign is a sentient being and more capable than any known AI. It's physical shell is a mammoth ship, at eight kilometers in length and it's massive main gun can break through any other Dreadnoughts kinetic barriers in a single shot. It has a massive element zero core which is capable of generating Mass Effect fields that are powerful enough to land a ship of it's unusual size on a planet. In addition to it's substantial warfare capabilities, Sovereign can also indoctrinate organics who are in it's company for extended periods of time.
Sovereign is voiced by Peter Jessop.
Sovereign was first sighted during the Geth attack on Eden Prime. For the majority of the game it was unknown who constructed Sovereign; the prevailing opinion was that it was constructed by the sentient machine race, the Geth. However, Shepard's encounter with Sovereign on Virmire shined new light on the existence of the Reaper race. Sovereign explained the 50,000 year cycle of indoctrination, extermination and genocide of any technologically advanced race. Their motives were never explained fully, only one motive is told; after the mass genocide the Reapers would harvest worlds bare of all technology and resources, probably so they could maintain themselves. The Reapers think all organic life is an accident, or a mutation. And that the geth are so primitive that the Reapers would have to destroy them for resources if they ever came
In an attempt to kick start this cycle again, Sovereign lead an all-out assault on the Citadel station. During the ensuing battle, Sovereign assumed direct control of Saren to unlock the station's Mass Relay and start the Reaper invasion. But Shepard stopped the Turian and temporarily stunned the Reaper in the process. The human Arcturus fleet exploited this sudden drop in the Reaper defense and the final killing blow was a torpedo from the Normandy that punctured Sovereign from bow to bow and caused a massive explosion.
Shepard saw the Battle of the Citadel as a sign of the inevitable war with the Reapers and warned the Council.
Mass Effect 2
Most of Sovereign was destroyed during the Battle of the Citadel and the few remaining pieces fell on the Wards. The debris was salvaged by Turians, who built their anti-material Thanix Cannon based on Sovereign's main gun. Cerberus also got their hands on several parts and used them to build EDI, the new Normandy's AI. In spite of all these pieces of advanced technology, the Council refused to acknowledge the Reaper threat and dismissed any claims of their existence.
Shepard's new crew member, Legion recounts the events of the first game from the Geth perspective. The arrival of Sovereign created a splinter among the Geth collective. Those who chose to follow the Reaper (or the Old Machines as the Geth call them), were branded as Heretics. Legion also reveals that the processes inside Sovereign identified it as "Nazara". However, it is unclear why the Reaper chose to call itself "Sovereign" at Virmire.
- There is a realm of existence so far beyond your own you cannot even imagine it. I am beyond your comprehension. I am Sovereign.
- Your civilization is based on the technology of the mass relays. Our technology. By using it, your civilization develops along the paths we desire. We impose order on organic life.
- You exist because we allow it, and you will end because we demand it.
- We are legion. The time of our return is coming. Our numbers will darken the sky of every world. You cannot escape your doom.
- I am the Vanguard of your destruction.