Reaper

The Reapers are a hyper-advanced machine race, rumoured to have existed fifty thousand years ago, but of whom no archaeological record remains. Mention of them was found in a data file, recovered from a geth's memory core by Tali'Zorah nar Rayya.

The Reapers were worshipped as gods by the geth, who believed that Saren Arterius was the prophet for the Reapers' return and that the mysterious Conduit was somehow the key to bring them back. Commander Shepard, driven by the vision from the Prothean Beacon, believed the Reapers were a viable threat to the galaxy, but the Citadel Council dismissed Shepard's claims as unfounded and the Reapers as a simple myth.

In fact, the Reapers -- a name bestowed by the Protheans; their true name is unknown -- are an extremely advanced race of synthetic warships that reside in the unexplored region referred to as dark space, the empty, starless space between galaxies. They hibernate there, dormant for thousands of years, before they are given the signal to return. Their origins are completely unknown. The only Reaper known to have communicated with organic life, Sovereign, claimed that the Reapers have neither beginning nor end - "we are eternal, the pinnacle of evolution".

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 eventually discover and use them – all part of their ingenious scheme to harvest the galaxy’s sentient life in a repeating cycle that has been looping for countless millennia.

While the other Reapers hibernate, a single Reaper remains behind, hidden in the depths of space. This vanguard can lie in a dormant state for thousands of years (the one period relevant to the game took approximately 50,000 years, but this can probably vary depending on chance and development speed of individual races), scanning the galaxy periodically for signs that intelligent organic life has advanced sufficiently. At which point their advance is sufficient is unclear, but once the vanguard decides the time is right, the subsequent Reaper attack is devastating.

The Cycle of Extinction
For reasons currently unknown, the Reapers cull the intelligent races of the Milky Way galaxy, returning to dark space with the organics' technology and resources. The Reapers leave no evidence of their conquest nor even of their existence – only desolate, barren ruins of those who came before.

The trap created by the Reapers was simple. A sentient species would develop FTL drive, but would still be limited in its speed. By leaving a network of relays capable of instant transport across the galaxy that led to the impressive Citadel, the Reapers ensured that it would become the center of galactic civilization. Further, Sovereign implies that the presence of the mass relays would lead the sentient species down a predetermined route with regards to weapons and armor technology (both of which are based upon element zero technology for the Citadel races). It is probable that this reduces the possibility of organic life discovering alternative, more advanced technology and progressing down a different 'path'; or, as Sovereign put it, "We impose order on the chaos of organic evolution."

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 sends a signal to the Citadel, instructing the keepers to activate the station’s hidden mass relay. This opens a path between the Citadel and dark space. The Reapers then flood through, killing the leaders of the assembled species before branching out and obliterating all spacefaring life around them.

Because the Reapers first enter the galaxy at the point that they have ensured will be the center of galactic politics, information and finance, they are able to cripple any resistance almost before the Citadel civilizations have any idea that they are under attack. The Citadel also gives them control of the relay network, cutting off star systems from each other and destroying communications.

The Reapers then use their control of the Citadel and its data to begin the most sinister phase of their attack. Records allow them to track down every settled planet and attack them, either strip-mining the worlds of resources or enslaving the populations with indoctrination and turning them into sleeper agents. Once they have harvested the galaxy, the Reapers wipe every trace of their existence from record and retreat back into dark space.

The Vanguard
The only Reaper to be witnessed by any living intelligent being after the Protheans became extinct was designated Sovereign. It was a colossal dreadnought, 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 Arterius, by those who encountered it.

Sovereign did act as transport for Saren and his geth minions – however, it was later discovered by Commander Shepard that Sovereign and its brethren were actually the masterminds behind the genocide. Saren began his search for Sovereign at the end of Mass Effect: Revelation using research stolen from Dr. Shu Qian, eventually finding the Reaper near or within the Perseus Veil.

Characteristics
Sovereign bore superficial resemblance to a cuttlefish, with a bulky semi-cylindrical body and a mass of metallic ‘tentacles’ extending from its front end. Its design appears to have influenced geth design (most notably of their dropships). This is not surprising, as the geth worship the Reapers as 'gods', considering them to be the epitome 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 insulted 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 the 'artifact' they found (see Mass Effect: Revelation), because Sovereign 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 and only forestalls the inevitable. Matriarch Benezia chose to die rather than risk falling under indoctrination again, and if Saren finally rebels against Sovereign he kills himself to prevent doing any further damage to the galaxy. This indoctrination is permanent (with the single possible exception of Shiala) and is one of the most insidious weapons of the Reapers.

The Reapers are immensely powerful and their technology is devastating. Sovereign’s destructive power was unrivalled in the known galaxy. Each of the 'tentacles' extending from its bow was equipped with a powerful directed energy beam which can sear through a cruiser in a single sustained burst. Its gigantic spinal-mounted gun was able to sear through the hulls of even the largest of capital ships with trivial ease, effortlessly penetrating their kinetic shields. Sovereign's defences included powerful shields that could block the projectiles of an entire fleet, along with an incredibly strong hull. Though they are sentient machines, the Reapers have habitable interiors that can transport a crew, either to help spread their indoctrinated slaves or to allow these slaves to tend to them, probably for both reasons. Speculation in the Codex suggests that each individual Reaper has a massive element zero core, allowing it to generate the staggering amounts of energy needed to land on a planet, and power its weaponry and shields.

However, the Reapers are not invincible. A concentrated effort by the fleets of organic races could destroy a Reaper (albeit with heavy losses) and this fact caused difficulties when the Protheans finally managed to strike a blow against the Reapers.

The Prothean Counterattack
After the last cycle, which obliterated the Prothean empire, a cadre of elite Prothean scientists hidden on Ilos survived the genocide. It took them decades to realise the connection between the Reapers, the Citadel, and the keepers, but this discovery gave them the key to breaking the cycle forever.

This plan hinged on the fact that the keepers have evolved, and now only respond to signals from the Citadel itself. As stated above, the Reaper vanguard signals the Citadel which in turn signals the keepers to open the mass relay, ushering in the next Reaper invasion. However, the Prothean scientists used a reverse-engineered prototype mass relay -- the Conduit -- travelled to the Citadel, and altered the Citadel signal. When Sovereign decided it was time to begin the cycle again, the keepers ignored the order.

This greatly complicated matters for Sovereign. In order to unleash its brethren from 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, a single Reaper would not be able to survive the combined might of the assembled Citadel races in a direct assault. It needed to find an agent that would lead it to the Conduit… an agent like Saren, who could comprehend the vision from the Prothean Beacon. With a fleet of geth ships, the Reaper launched an all-out assault on the Citadel, nearly devastating the entire fleet.

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. Shepard knew the Reaper fleet, though dormant and hibernating, was still out in dark space and vowed to find some way to stop them.

Trivia

 * The Inhibitors in the Revelation Space novels, by Alastair Reynolds, bear many similarities with the Reapers.
 * The idea of a terrifying and incomprehensible alien intelligence waiting in the depths of space, is a feature of Lovecraftian horror.