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, its 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 construct, while others believe it is a Prothean relic. Its design, 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.