Sovereign: the Reaper vanguard the geth called Nazara

Sovereign was a Reaper that for a time served as the apparent flagship of the rogue Spectre Saren Arterius. An enormous dreadnought two kilometers long, larger than any ship in any known fleet, it was crewed by geth and krogan and carried a spinal-mounted main gun likely capable of cracking a dreadnought's kinetic barriers in a single shot. Most who encountered it believed it a geth construct or a Prothean relic, but Commander Shepard discovered it was a fully sentient Reaper, the vanguard of a species waiting in dark space. (Mass Effect)

Nature and purpose#

Shepard at first assumed Sovereign was a Reaper vessel that Saren had found and reactivated, but it was in truth an actual Reaper, a fully sentient and immensely powerful artificial intelligence. The geth obeyed it because they saw it as the highest form of synthetic evolution and worshipped it as a god, though Sovereign held nothing but contempt for the worship they hurled at it and regarded Saren and the geth alike as tools. To the geth it identified itself by the name Nazara; "Sovereign" was a title coined by Saren. (Mass Effect; Mass Effect 2)

As the Reapers' vanguard, Sovereign had spent some fifty thousand years in near-constant hibernation, waking periodically to assess the development of organic races. Its duty was to bring about the next turn of the cycle by signaling the keepers on the Citadel to open the station's hidden mass relay to dark space, ushering its kind through. Like all Reapers it could subjugate organic minds through indoctrination, surrounding itself with a subtle field that reduced its victims to obedient shells. (Mass Effect)

Discovery and the alliance with Saren#

Years before Saren went rogue, a survey team working for the batarian entrepreneur Edan Had'dah found Sovereign orbiting an uncharted world near the Perseus Veil. Had'dah recruited the defected Alliance scientist Dr. Shu Qian to study it, but its indoctrination was already at work on them both. The Spectre Saren Arterius, investigating an attack on the Sidon facility, grew intrigued, killed Qian and Had'dah, and took their research to find the ship for himself. (Mass Effect: Revelation; Mass Effect)

Because the keepers no longer answered its signal, owing to ancient Prothean sabotage, Sovereign needed an organic ally to learn what had gone wrong, since revealing itself openly would have united the galaxy against it. It made an empty alliance with Saren, who believed he could prove organics useful to the Reapers and so spare them. Saren took Sovereign as his flagship and was named its prophet to the geth, but he too fell under its indoctrination, kept just intact enough to hunt for the Conduit, a Prothean backdoor to the Citadel. (Mass Effect; Mass Effect 2)

The road to the Citadel#

When a Prothean Beacon surfaced on the human colony of Eden Prime, Sovereign brought Saren and an army of geth to seize it, devastating the colony with weapons and engine wash alike before the marines' distress call alerted the Normandy. Later, at Saren's krogan breeding base on Virmire, Sovereign spoke to Shepard directly, voicing its disdain for organic life, but Shepard destroyed the base and escaped before it arrived. The encounter left Saren doubting, so Sovereign implanted him with cybernetics to deepen its control. (Mass Effect)

With the aid of other indoctrinated victims, Saren found the Conduit on Ilos and used it to reach the Citadel with a force of geth, seizing the station from within. Sovereign meanwhile led a vast geth armada against the Citadel Fleet, positioned itself at the Citadel Tower, and began to override the controls and open the relay to dark space. (Mass Effect)

The fall of the vanguard#

Shepard's squad pursued Saren through the Citadel. When Saren died, either in battle or by his own hand if Shepard persuaded him to break free of the Reaper's grip, Shepard used a data file made by the Prothean virtual intelligence Vigil to delay Sovereign's takeover. In a last bid Sovereign reanimated Saren's corpse through its implants and linked its own consciousness to it, fighting Shepard through that avatar while devastating the Alliance fleet outside. When the avatar was destroyed, the link corrupted and Sovereign's shields failed; under the full fire of the Systems Alliance fleet it was destroyed by a fatal shot from the Normandy, averting the return of the Reaper fleet. (Mass Effect)

For all the Council's relief, Shepard recognized that Sovereign had been only a vanguard, and that the rest of the Reapers still waited beyond the galactic rim. Sovereign's fall is the climax of the Mass Effect 1 story. (Mass Effect)

Aftermath#

Most of Sovereign's debris rained onto the Wards, and between rescue crews and the keepers little could be studied, leaving the Council unconvinced it had been more than a geth construct. Turian researchers did recover enough to learn that its main gun fired a stream of molten metal by electromagnetic field, and reverse-engineered the weapon as the Thanix cannon, while Cerberus recovered pieces for the AI aboard the new Normandy, to serve as anti-Reaper algorithms. It was also implied that Sovereign, or another Reaper acting on the same objectives, had indoctrinated the rachni queens, provoking the aggression that sparked the Rachni Wars. (Mass Effect; Mass Effect 2)