Sovereign is the flagship of the rogue Spectre Saren. An enormous dreadnought larger than any other ship in any known fleet, Sovereign is crewed with both geth and krogan. At two kilometers long, it's spinal-mounted main gun is likely capable of penetrating another dreadnought's kinetic barriers with a single shot.
How Saren acquired this incredible warship is unknown. The prevailing opinion is that Sovereign is a geth constrict, while others believe it is a Prothean relic. Its deign, however, hints at a more alien and mysterious origin.
The attack on Eden Prime demonstrated Sovereign's ability to generate mass effect fields powerful enough to land on a planetary surface. This implies it has a massive element zero core, and the ability to generate staggering amounts of power.
Mass Effect 2
The flagship of the rogue ex-Spectre Saren Arterius, Sovereign is a devastating dreadnought of unprecedented power.
At two kilometers long, Sovereign was believed to have been powered by a gigantic element zero core, the only energy source potent enough to land the massive vessel on a planet.
During the Eden Prime War, the ship unleashed its peerless destructive force against which Citadel Council ships were defenseless. With a virtually indestructible flagship and a crew of fanatic geth and krogan, Saren nearly succeeded in wiping out his enemies. Only the resourcefulness and bravery of the commander and crew of the SSV Normandy stopped him.
The overwhelming power of Sovereign ignited bizarre speculation in tabloid media that the vessel was sent by extragalactic invaders, or was -- despite indeterminate photographic evidence at best -- a single giant robot. Some source even claimed that Sovereign was alive. Such conjecture, however baseless, remains to this day.
Mass Effect 3
Sovereign was the first Reaper encountered by the modern Citadel races. Military leaders initially assumed that Sovereign was a geth or Prothean flagship commanded by Saren Arterius, a Rogue Spectre. The truth was far more alarming. The massive ship was itself intelligent, and Saren proved to be under its control.
The attacks by Sovereign against Eden Prime and the Citadel removed any uncertainty about the Reapers' technological superiority. Sovereign's formidable shielding and firepower allowed it to hold off the combined fleets of the Citadel, and its mass effect fields proved powerful enough to let the enormous vessel land on a planet's surface.
Sovereign's mission--to open a mass relay that would transport the other Reapers from dark space--proved its undoing. During the Battle of the Citadel, Sovereign linked its consciousness to Saren's. When Saren's death corrupted the signal and shut down the Sovereign's shields, Sovereign's destruction soon followed.