Sovereign also revealed to Commander Shepard that the Reapers were the original creators of the Citadel and the mass relay network, though the commander apparently failed to reveal this information to the galaxy at large or it simply never took, as it was never integrated into Codex databases distributed amongst the Council races. Smaller pieces of their technology also exist throughout the galaxy. The mass relays exist so that any intelligent life in the galaxy would eventually discover them and base their technology upon them – all part of a scheme to harvest the galaxy’s sapient life in a repeating cycle of purges that has continued relentlessly over countless millions of years.
A common myth to several cultures in the galaxy, Reapers were once imagined as space monsters that consumed entire stars. Archaeologists who searched for the sources of such myths found little besides the themes of all-consuming devils that are common to primitive cultures.
Although accurate information about the Reapers remains scarce, the galaxy now knows that the Reapers are not a myth—they are a real and devastating threat, though, still not perfect.
The Reapers are technologically superior to the organic species of the galaxy—but the degree of that superiority is a matter of debate in the intelligence community.
The Reapers' thrusters and FTL drives appear to propel them at more than twice the speed of Citadel ships. Estimates of their location in dark space suggest they can travel nearly 30 light-years in a 24-hour period. Reaper power sources seem to violate known physical laws. Reapers usually destroy fuel infrastructure rather than attempting to capture it intact, indicating that Reapers do not require organic species' energy supplies. Consequently, the Reapers attack without regard for maintaining supply lines behind them, except to move husks from one planet to another. Unlike Citadel ships, Reapers do not appear to discharge static buildup from their drive cores, although they sometimes appear wreathed in static discharge when they land on planets.
The main gun on a Reaper capital ship dwarfs that of the Alliance's Everest-class dreadnoughts. No dreadnought has yet survived a direct hit from the weapon. Estimates put its destructive power anywhere from 132 to 454 kilotons of TNT. Even if the target is hardened, as in the case of a surface-based missile silo, the gun can instead bury the target beneath molten metal. Precise targeting computers and correctors also give Reaper weapons a longer effective range than organics' dreadnoughts or cruisers.
The kinetic barriers on a Reaper capital ship can shrug off the firepower of a small fleet. Weapons specifically designed to overcome shields, such as the Javelin, GARDIAN lasers, or the Thanix series, can bypass the barriers to some degree. The difficulty is getting close enough to use them—the surface-mounted weaponry on Reaper ships, similar in principle to GARDIAN, presents an effective defence against organic species' fighters.
Although clearly technologically superior to the Citadel forces, the Reapers have experienced casualties in the battles across the galaxy (as of 2186). This indicates that, theoretically, with the right intelligence, weapons, and strategy, the Reapers could be defeated.
Unlike the mass effect relays that they created, Reapers do not have quantum shields. Locking itself down at a quantum level would leave a Reaper unaware of its surroundings until the shielding deactivated. Instead, Reapers rely on kinetic barriers.
In the case of a Reaper capital ship, these kinetic barriers can hold off the firepower of three dreadnoughts simultaneously, although prolonged fire can deplete them. Unless extenuating circumstances occur, four dreadnoughts can destroy a Reaper capital ship. Simply ganging up on them is not a viable long-term strategy however, since the number of Reaper capital ships is far greater than the number of dreadnoughts available to the species of the galaxy. Weapons designed to maximise heat damage, such as the Thanix series, show better results against the Reapers than pure kinetic impacts, allowing even single dreadnoughts to inflict heavy damage if allowed to bring their upgraded weapons to bear.
The barriers of a Reaper destroyer are less formidable than those of a capital ship. It is possible for a single cruiser or many fighters to disable or demolish a destroyer if they can get within range before they are themselves destroyed.
The Reapers' energy sources are not infinite. For example, to land on a planet, a Reaper must substantially reduce its mass. This transfer of power to its mass effect generators leaves the Reaper's kinetic barriers at only partial strength.
Sovereign was destroyed while assuming direct control over Saren. The feedback from Saren's death seemed to entirely overload Sovereign's shields. Current Reapers (as of 2186) do not seem to suffer from this design flaw.
Reaper capital ships can turn faster than Citadel dreadnoughts, but to do so, they must lower their mass to a level unacceptable in combat situations. Consequently, it is possible for a dreadnought to emerge from FTL travel behind a capital ship, then bring its guns to bear faster than the Reaper can return fire. This is a poor tactic, however, against Reapers flying in proper formation.
Reaper "indoctrination" is an insidious means of corrupting organic minds, "reprogramming" the brain through physical and psychological conditioning using electromagnetic fields, infrasonic and ultrasonic noise, and other subliminal methods. The Reaper's resulting control over the limbic system leaves the victim highly susceptible to its suggestions.
Organics undergoing indoctrination may complain of headaches and buzzing or ringing in their ears. As time passes, they have feelings of "being watched" and hallucinations of "ghostly" presences. Ultimately, the Reaper gains the ability to use the victim's body to amplify its signals, manifesting as "alien" voices in the mind.
Indoctrination can create perfect deep cover agents. A Reaper's "suggestions" can manipulate victims into betraying friends, trusting enemies, or viewing the Reaper itself with superstitious awe. Should a Reaper subvert a well-placed political or military leader, the resulting chaos can bring down nations.
Long-term physical effects of the manipulation are unsustainable. Higher mental functioning decays, ultimately leaving the victim a gibbering animal. Rapid indoctrination is possible, but causes this decay in days or weeks. Slow, patient indoctrination allows the thrall to last for months or years.
Even with all the Reapers' power, harvesting every sapient species in an entire galaxy can take decades or even centuries. The most time-consuming part of the process is gathering DNA from the population. To accelerate the effort, the Reapers follow a consistent procedure.
Victims who cooperate, surrender, or are captured by husks are sorted into camps. It is believed the husks possess receptors that allow them to analyse a victim's DNA through sight, smell, or touch. Victims that meet their standards are herded from the camps into processor ships. Those the husks deem insufficient are either turned into husks themselves or indoctrinated to serve as slave labour. The Reapers use this last option to give their victims false hope—many captives who would otherwise fight back become docile when they see members of their own kind obey and survive.
The processor ships reduce victims to a transportable liquid. Like in a slaughterhouse, the ships' design prevents victims from seeing or hearing what happens elsewhere so that they do not panic. The victims are ushered into locking pods, then rent apart and dissolved into paste that is flushed to storage vats.
The rate of killing is phenomenal. Intelligence estimates suggest there are more than 400 processor ships on Earth (in 2186), killing approximately 1.85 million humans per day. In combination with battlefield deaths, disease, and famine, this pace will result in the complete depopulation of Earth within a decade. As the husks and indoctrinated slaves build more slaughtering facilities, the kill rate can only increase.
The Citadel races have classified the known variants of Reapers into four types:
- CAPITAL SHIPS are Sovereign-class Reapers two kilometres in length. They typically target the dreadnoughts, defence installations, and industrial cities of organic civilisations. Experts believe the Reapers harvest a single species of organics during each cycle of extinction to create these massive ships. Some capital ships are capable of launching small drones equivalent to fighters.
- DESTROYERS are 160 meters and, in astounding numbers, make up the bulk of the Reaper fleet. They engage cruisers and other, smaller ships, as well as communications posts and enemy command centres. Research suggests destroyers are created from those species that are not harvested to make capital ships.
- TROOP TRANSPORTS carry husks to unconquered planets and bring victims of the harvest to Reaper processing centres. They vary in length from 200 meters to one kilometre, but, unlike capital ships and destroyers, do not appear to be self-aware. Instead, other Reapers operate troop transports remotely.
- PROCESSORS, also called "slaughter ships," are mobile centres for mass DNA harvesting. Like troop transports, processors appear to be remotely operated by sapient Reapers.
Capital ship Reapers bear an appearance based on the race it harvests, for example, Harbinger is based on the Apex race, the ones who built the reapers. with a two-kilometre long, bulky and semi-cylindrical body, a tapering plate over the rear and a mass of metallic "tentacles" extending from its front end, in addition to six jointed legs extending from its body. When the Reaper fleet is revealed in dark space they are all shown with this basic design (however, one Reaper in the foreground shown when Harbinger awakens the fleet seems to resemble a scarab beetle) but with great diversity in tentacle number, shape and orientation, some with extended heads and others (particularly Harbinger) having multiple glowing eyes. This diversity is presumably due to the Reapers' reproduction method, in which vast numbers of a single species are harvested, melted down into a raw genetic paste, and then used to construct a "larva" that takes on the characteristics of the species from which it was created. The core of the Reaper is in the image of the species that was used to create it while the exterior follows a similar design that is most efficient for their purpose.
There seem to be a few 'classes' of Reaper, such as the capital ships (of which Sovereign was one), and the destroyers, which have legs and move across the ground. These have a beetle-like head and their main weapon is located just above this. These are only around 160 m tall, but still formidable opponents.
Sovereign's design appears to have influenced the 'heretic' geth. This is not surprising, as these geth worship the Reapers as "gods", considering them to be the epitome of independent AI. Wherever the heretic geth become entrenched they build monuments to the Reapers resembling Sovereign's 'tentacles' around a glowing orb, and genuflect. 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.
Even without their indoctrinating influence, Reapers are immensely powerful warships 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 "magnetohydrodynamic" weapon which ejects a stream of molten metal at a fraction of the speed of light which could tear through a cruiser in a single sustained burst. Its gigantic spinal-mounted gun was able to rip through the hulls of even the largest of dreadnought-class ships with ease, effortlessly penetrating their kinetic barriers. Sovereign's defences included powerful shields that could block the projectiles of an entire fleet, along with an incredibly strong hull. Though they are sapient 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—in truth it is probably a combination of both, and Reapers have been witnessed to possess Dragon's Teeth within their interior. Speculation in the Codex suggests that each individual Reaper has a massive element zero core (proven true on the Derelict Reaper found by Commander Shepard) which, coupled with the likely enormous quantities of energies at its disposal, allows it to generate the staggering mass effect field needed to land on a planet.
However, the Reapers are not invincible. When the Reapers go into states of hibernation between cycles, they are vulnerable. By taking refuge in dark space, the Reapers ensure they will not be discovered by accident and destroyed while they wait for their vanguard to open the Citadel mass relay. A concentrated effort by the fleets of organic races could also destroy a Reaper even if it is at full power: a single fleet managed to destroy Sovereign when its shields and weapons were disabled.
Aside from their formidable firepower and indoctrination, Reapers are also capable of mustering up their own ground forces to attack their enemies: Husks. Husks are aggressive, mindless drones, created from organic beings who are exposed to Reaper technology, such as Dragon's Teeth, resulting in their organic bodies being consumed by cybernetics. Husks are used as shock troops and swarm enemies in waves. Husks can be modified and combined to create Abominations, Scions, and Praetorians.
The Reapers have been around for billions of years—the derelict Reaper discovered by Commander Shepard was 37 million years old.
The cycle of extinction has been going on for as long if not longer, ever since the Reapers were created. They laid a trap for the races of the galaxy in the form of the mass relays, which offered instant transport around the galaxy (leading to the impressive Citadel), almost certainly better than the FTL technology the discoverers would possess. This channelled the development of the race's technology, possibly so that it was more likely to be usable by the Reapers when they returned, though it also causes the races to generally only develop tech that is significantly weaker than modern Reaper tech, as seen by Reapers being able to move twice the speed of any Alliance vessel. This path also made element zero-based weapons and armour technology more probable.
The Reapers created or enslaved the Keepers as guardians of the Citadel, and programmed them to maintain the Citadel while also preventing the residents from learning its secrets. When the vanguard left by the Reapers in the galaxy broadcasted a signal, the Keepers would activate the station's mass relay, bringing in the Reapers from dark space. As the Citadel would be the centre of galactic finance, information and society, the Reapers controlling it would allow them to find all worlds and settlements of the species there. They will also have control of the relay network from here.
After harvesting the galaxy, they remove all traces of their presence and go back to dark space.