You represent chaos. We represent order. Every organic civilization must be harvested in order to bring order to the chaos. It is inevitable. Without our intervention, organics are doomed. We are your salvation.
The Reapers are a highly-advanced machine race of synthetic-organic starships. The Reapers reside in dark space: the vast, mostly starless space between galaxies. They hibernate there, dormant for fifty thousand years at a time, before returning to the galaxy.
These giant machines are ancient; their true name is unknown. "Reapers" was a name bestowed by the Protheans, the previous galactic power fifty thousand years before, and the geth refer to them as the Old Machines. In the end, the Reapers spare little concern for whatever labels other races choose to call them, and merely claim that they have neither beginning nor end.
The Reapers are the original creators of the Citadel and the mass relay network. These massive constructs 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 sentient life in a repeating cycle of purges that has continued relentlessly over countless millennia.
A Reaper is essentially "billions of organic minds, uploaded and conjoined within immortal machine bodies."
In terms of physical design, Reapers bear superficial resemblance to a cuttlefish or squid, with a bulky semi-cylindrical body, a tapering plate over the rear and five tentacle-like "legs" or arms extending from its front end, in addition to six jointed legs extending from its body. The rear-most of the larger legs have crescent-shaped extensions. Colossal in size, Reapers are known to range from 160 meters to over 2 kilometers in length.
The core of any Reaper is constructed in the image of the species that was harvested to create it, while the exterior follows a standardized design that is most efficient for their purpose.[1] When the Reaper fleet is revealed in dark space, they are shown to all share the same basic design, but with great diversity in limb number, shape and orientation, some with extended heads and others having multiple glowing lights. This may illustrate the variety of Reaper subtypes, as it is revealed during the Reaper invasion of the galaxy that several different varieties of Reaper exist.
While the outward appearance varies between subtypes, all of them retain features of their forebears, the Leviathans, an ancient aquatic race that dominated the galaxy in ages past. The largest known Reaper ships essentially copied the Leviathan form while the smaller ones diverge to varying degrees.
Harbinger: The first Reaper ever created, Harbinger appears to be of a unique subtype; Alliance intelligence identifies it as being the largest Reaper in the armada, and its design differs notably from the Capital Ship subtype, having only four main legs and multiple glowing "eyes". Exactly how much more powerful it is than a normal Reaper capital ship is unknown.
Capital Ship: Also known as Sovereign-class, the two-kilometer-long Capital Ships are the most well-known Reaper subtype. Their main weapon is a spinal mounted "magnetohydrodynamic" cannon with a yield of 132 to 450 kilotons of TNT, which dwarfs the main gun of an Everest-class Alliance dreadnought. No known ship, not even a dreadnought, has been known to survive a hit from this weapon. Capital Ships are also armed with multiple cannons in their "tendrils" capable of shearing through most opposing vessels in a single hit, a point defense system (similar to the GARDIAN systems favored by organics) for anti-fighter and anti-projectile purposes, and are capable of unleashing swarms of Oculus drones to engage fighter-sized craft. They are extremely durable, capable of taking the continuous and simultaneous fire of four dreadnoughts before they start to lose their kinetic barriers.
Troop Transport: Troop Transports vary in length between 200 meters and one kilometer and are used to transport husks to unconquered worlds and bring victims to Reaper processing centers. They lack sentience, being remotely controlled by the Reapers.
Processor: Mobile centers for mass DNA harvesting. Like the Troop Transports, Processors lack sentience, being controlled remotely by the actual Reapers.
Destroyer: Destroyers are only 160 meters in height, but possess a formidable capacity for destruction despite their reduced stature, with their main gun easily capable of destroying cruisers in seconds. Unlike Capital Ships, they have four main legs, along with five jointed appendages encircling their "head". The frontal plates of a Destroyer can fold to the sides, exposing a powerful beam weapon. They are used to escort the capital ships, destroying smaller targets such as frigates. On the ground, they are capable of unfolding their legs and walking to support the husks, becoming extremely powerful heavy walkers. They are nigh-immune to ground vehicle fire and not at all fragile in space, but a cruiser is noted to be able to take out a Destroyer fairly quickly.
Reapers and their technology have been observed to exert a disturbing influence on organic beings. This mental manipulation is known as indoctrination. Put simply, any organic being who is in close proximity to a Reaper or certain Reaper artifacts 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 no longer capable of independent thought.
Reapers can control the rate of this process. Optimally, the subject is led to believe it is still acting on its own convictions. Indoctrination can drive people mad outright, and people deemed useful by the Reapers are given just enough free will to remain competent at their tasks.
This indoctrination is permanent, almost impossible to subvert, and is one of the most insidious weapons of the Reapers. Entire civilizations can be delivered into the Reapers' hands by the indoctrination of a few influential individuals. During the invasion of Earth, the Reapers broadcast messages inviting diplomats into their holds to negotiate, where they would then presumably be indoctrinated.
A Reaper's indoctrination field can remain active even if it is largely disabled and incapable of action. A Cerberus science team was indoctrinated by being inside a Reaper that had otherwise been floating derelict for 37 million years, its only obvious activity being mass effect field generation.
Though they are sentient entities, the Reapers have habitable interiors that can transport organic or other smaller lifeforms, possibly to help spread their indoctrinated slaves or to allow these slaves to tend to them.
Even without their indoctrinating influence, Reapers are immensely powerful warships and their technology is devastating. One armament common with the various subtypes is a powerful "magnetohydrodynamic" weapon which ejects a stream of molten metal at a fraction of the speed of light, capable of tearing through a cruiser in a single sustained burst. The gigantic spinal-mounted gun of capital-class Reapers is able to rip through the hulls of even the largest of dreadnought-class ships with ease, effortlessly penetrating their kinetic shields. Reaper defenses include powerful shields that could block the projectiles of an entire fleet, along with incredibly strong hulls.
Reapers can reach FTL velocities much greater than anything the current civilizations of the galaxy are capable of. They are able to generate mass effect fields of incredible strength, allowing them to land on planetary bodies and make changes of direction that would usually be physically impossible for ships even approaching their size. These capabilities are likely powered by immense element zero cores, though other sources of power generation may also be involved.
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.
Reapers recognize each other through a complex Identify Friend/Foe system. Reaper interrogation signals do not simply look for a friendly transponder code, they look for a friendly intelligence. If other races attempted to steal the IFF for themselves, the IFF comes with a virus that can potentially disable any system it's connected with.
The Reapers are known to use highly advanced computer viruses for other purposes as well, such as to subvert synthetic species that do not willingly join them in their conquest of organic civilizations. Synthetic intelligences would eventually be harvested anyway as they are not a part of the Reapers' ultimate goals. Reaper viruses are not unbeatable; on at least one occasion an unshackled artificial intelligence was able to counteract one.
Aside from computer viruses, the Reapers employ other forms of microtechnology, a notable example being nanides, or microscopic robots. Reaper nanides are used in the harvesting of organic species, often deployed through devices such as dragon's teeth to convert organic beings into the mindless, aggressive cyborg husks. Husks are used as ground forces and shock troops by the Reapers, overwhelming and breaking the morale of organic races who are forced to witness their own kind being used against them. This doubles as an efficient method of depleting enemy numbers while simultaneously replenishing Reaper armies quickly.