Sovereign is a colossal alien starship of incredible power that appears in Mass Effect. At first appears to be commanded by Turian ex-Spectre Saren Arterius, but in actuality it is one of the ancient Reapers that destroy all intelligent life in the galaxy every 50,000 years.
Sovereign is the vanguard of the Reapers, left behind in the Milky Way after the other Reapers retreat to dark space. It remains to watch the development of the galaxy's organic races and usher in the next Cycle when the time comes. Sovereign's mission to usher in the next cycle was threatened after the Protheans - the dominant race of the last cycle - had used their Conduit to secretly board the Citadel after the Reapers had left and disable the signal Sovereign used to command the Keepers to open the Citadel mass relay. By the time the next cycle was due, the Citadel had been re-inhabited by several races and was defended by a large fleet, one that could still threaten Sovereign with destruction despite the Reaper's power. With the extinction cycle delayed, Sovereign began working towards retaking the Citadel through indoctrinated agents that had been swayed to its control through Reaper artifacts scattered across the galaxy. One such agent, the Turian Spectre known as Saren, assisted Sovereign in contacting the machine race known as the Geth. Many of the Geth saw Sovereign (or as they called it, "Nezara", a Quarian word meaning "supreme ruler") as a machine god, the pinnacle of synthetic evolution, and worshipped it as their deity. This created a rift in the Geth consensus and these "heretics" were banished from the Perseus Veil. They pledged themselves to Saren and Sovereign and joined them in attacking the human colony of Eden Prime as well as several other planets where they might find clues to the whereabouts of the Conduit.
When Systems Alliance officer Commander Shepard was assigned to hunt down Saren, he would discover Sovereign's true nature after communicating with it on Virmire, Saren's home base. The Reaper revealed to Shepard the truth about the Reapers and the cycle of genocide that was soon to begin again. After the Conduit was found on Ilos, Sovereign led the Geth fleet in an attack on the Citadel while Saren used the Conduit to board the station and disable it from within. Sovereign's might proved to be too great for the Citadel Defence Fleet and destroyed many of their ships as it approached its main target. However, Shepard had followed Saren through the Conduit on Ilos and boarded the Citadel, eventually reaching the Presidium and confronting and killing Saren. With Saren dead, Sovereign possessed his corpse, shedding his flesh and revealing a monstrous mechanical skeleton. The creature attacked Shepard's squad in a fierce battle, but was eventually destroyed. The signal Sovereign used to possess Saren's body fed back against it when the body was defeated, disrupting the Reaper's defences. Left wide open, Sovereign was vulnerable to the Citadel/Alliance fleet and was obliterated by their combined firepower.
- FTL Spaceflight - Sovereign is capable of achieving speeds exceeding the speed of light. Despite its size, it can easily catch up to and outmaneuver smaller vessels.
- Mass-effect fields - Reapers are all equipped with element-zero generators that allow them to create mass effect fields more powerful than those of any organic-designed spacecraft. The mass effect fields can repel enemy firepower and allow Sovereign to perform maneuvers that would tear apart other ships of its size. Sovereign is also capable of making planetfall thanks to the mass effect fields it generates and can operate on a planetary surface for any length of time.
- Indoctrination - Like all Reaper ships and tech, Sovereign can corrupt and control organic minds by subjecting them to certain energy fields and/or ultrasonic and infrasonic noise, planting subtle ideas and commands into subjects' minds that they feel compelled to follow without fully understanding why. The longer any thrall is subjected to indoctrination, the less effective a servant they become as the process slowly destroys the mind over time, eventually killing the subject or leaving them in a vegetative state.
- Weapons - Sovereign is armed with mass-accelerator cannons and magnetohydrodynamic launchers that fire molten metals at such speeds that they will punch through ship barriers and hulls with ease.