Reaper

The Reapers are an extremely advanced race of synthetic AI’s that lurk inside the unknowable region referred to as dark space; the black, lifeless void between galaxies. Their origins are completely unknown - the Reapers claim no one built them, that they are eternal and the peak of evolution.

Technology and the Cycle of Extinction
The Reapers were the original creators of the Citadel and the mass relays. They created these massive stations so that any intelligent life in the galaxy would discover and use them – all part of their ingenious and horrifying scheme to harvest the galaxy’s sentient life in a repeating 50,000 year cycle that has been looping for countless millennia. For reasons unknowable, the Reapers (as they were dubbed by the Protheans, their true name is unknown, if they even possess one) cull the intelligent races of the Milky Way galaxy on this timeline, taking with them all technology and resources and leaving no evidence of their conquest – only desolate, barren ruins of those who came before us. After they strip the galaxy bare, they retreat back into the void of dark space, leaving only a single vanguard (Sovereign) behind, who will lie in a dormant state for 50,000 years, scanning the galaxy periodically for signs of intelligent life. Once the sentient races have established themselves on the Citadel with the aid of the keepers; an organic race either created or enslaved by the Reapers in order to maintain the Citadel’s basic functions without revealing its secrets, the vanguard will send a signal to the Citadel instructing the keepers to activate the station’s hidden mass relay – opening a junction between the Citadel and dark space. The Reapers would then flood through, killing the leaders of the assembled species and then branching out and obliterating all sentience around them in calculated and methodical manner before wiping their existence from record and retreating back into the black veil surrounding our galaxy, and the cycle begins anew.

The Vanguard
The only Reaper ever to be witnessed by any living intelligent being post-Prothean era was designated Sovereign. It was a colossal juggernaut, several times the size of any known vessel – even dwarfing the massive asari flagship, Destiny Ascension. At first, it was presumed to be the flagship of the rogue Spectre agent, Saren, by those who encountered it. Indeed, Sovereign did act as transport for Saren and his geth minions – however, it was later discovered by Commander Shepard that it was, in fact, Sovereign who was truly the mastermind behind the genocide. Sovereign, and presumably all Reapers, created a mind-altering effect in those who were subjected to prolonged exposure to the vessel. This process was known as indoctrination – the wills of those around Sovereign were slowly eroded until their free will (and often sanity) shattered, and they became the mindless and obedient slaves of the Reaper. It was in this way that Saren and his servants, such as Matriarch Benezia, were controlled.

Sovereign’s destructive power was unrivaled in the known galaxy; its gigantic spinal-mounted gun was able to sear through the hulls of even the largest of capital ships with trivial ease, going straight through their kinetic shields. It bore superficial resemblance to a squid, with a bulky semi-cylindrical body and a mass of metallic ‘tentacles’ extending from its front end.

The Prothean Counterattack
After the last cycle, which obliterated the Prothean empire, the last remaining cadre of elite Prothean scientists hidden on Illos perfected the technology of the mass relays, and using this knowledge, they constructed what they referred to as the Conduit; essentially a ‘back-door’ entrance to the Citadel that was linked to Illos. The fate of the scientists after they went through the Conduit is unknown; however, in their absence, the keepers have deviated from the protocol originally set by the Reapers; they now only respond to instructions from the Citadel itself, rather than an outside signal. This greatly complicated matters for the current Reaper vanguard, Sovereign. In order to unleash its brethren in dark space, it would have to find a way to manually activate the relay from inside the Citadel. While Reapers are undoubtedly beings of terrible power and ferocity, even one of them would not be able to survive the combined might of the assembled Citadel races in a direct assault. So, it needed to find an agent that would lead it to the Conduit…an agent like Saren.

Fortunately, Saren was stopped by Commander Shepard and company, and Sovereign was destroyed. How long this will stall the eventual and inevitable return of the Reapers remains to be seen.

The geth worship the Reapers as 'gods', considering them to be the perfection of independent AI. Wherever the geth become entrenched they build 'monuments' to the Reapers that resemble the 'tentacles' on Sovereign around a glowing orb, and bow down to them. Sovereign was apparently disgusted by the adoration of such simple, base synthetics, but it did see their value as pawns and possible replacements for the flawed and organic keepers.

Reapers and their technology have a strange effect on organic beings. Both Dr Shu Qian and Edan Had'dah began acting oddly after coming into contact with a possible Reaper artifact (see Mass Effect: Revelation), and Sovereign itself produces an effect known as indoctrination. Put simply, any organic being who is aboard Sovereign for too long comes to believe the Reapers are correct in their goals and will do anything to serve them. Gradually the mind is eroded until the individual becomes a mindless slave, unless they can prove their value. Only beings of immense mental strength such as asari matriarchs can resist this indoctrination but even then, their resistance only lasts a short time. This indoctrination is permanent (with the single possible exception of Shialla).