Reaper

The Reapers, known by the geth as the Old Machines, 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 they are given the signal to return. Their origins are completely unknown; the first Reaper known to have communicated with organic life, referred to by Saren Arterius as Sovereign, claimed that the Reapers have neither beginning nor end. Even their true name is a mystery; "Reapers" was a name bestowed by the Protheans, and Sovereign stated that whatever the Protheans chose to call them was of little concern to its kind.

Sovereign also reveals that the Reapers were 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.

Design
Sovereign, the first Reaper encountered, bore a 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. When the Reaper fleet is revealed in dark space they are all shown with this basic design but with great diversity in limb number, shape and orientation, some with extended heads and others (particularly Harbinger) having multiple glowing eyes; 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. 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.

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.

Subtypes
There are several varieties of Reaper, which vary significantly in size, purpose and firepower. The most well-known subtype are the two-kilometer long Sovereign-class "Capital Ship" Reapers. Capital Ships are armed with multiple "magnetohydrodynamic" cannons capable of shearing through most opposing vessels, and are also capable of unleashing swarms of Oculi to engage fighter-sized craft.

The Reaper known as 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 Sovereign-class subtype. In addition, Harbinger's cannons appear to be capable of a much higher rate of fire than those of any other Reaper.

In addition to Reapers on the scale of Sovereign, there is a subtype of Reaper known as the Destroyer. Destroyers are only 160 meters in height, considerably smaller than the two kilometers that Sovereign boasted, but possess a formidable capacity for destruction despite their reduced stature. The frontal plates of a Destroyer can fold to the sides, exposing a powerful beam weapon.

Other subtypes of Reapers include Troop Transports and Processors, both of which lack sentience and are instead remotely controlled by other Reapers. 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. Processors are mobile centers for mass DNA harvesting.

Indoctrination
Reapers and their technology have been observed to exert a disturbing influence on organic beings. Both Dr. Shu Qian and Edan Had'dah began acting oddly after coming into contact with the "artifacts" they found. 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. Sovereign could partly control the rate of this process as seen with Saren; in order for him to serve the Reaper efficiently, Saren needed a measure of free will. This eventually convinced him that the only way to preserve organic life was to submit to the Reapers provided they could prove themselves useful.

Only beings of immense mental strength, such as asari matriarchs, can resist indoctrination, and 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. Even more horrifying is that the indoctrination field remains active even if the Reaper 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.

Technology
Even without their indoctrinating influence, Reapers are immensely powerful warships and their technology is devastating. Sovereign’s destructive power was unrivaled 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 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 both. Speculation in the Codex suggests that each individual Reaper has a massive element zero core 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. Occasionally, to bolster husk defenses the Reapers will employ devices known as Barrier Engines that cocoon individual husks in a durable biotic barrier.

The Cycle of Extinction
According to the Catalyst, the Reapers destroy advanced organic life in order to preserve a fragile balance. The Reapers believe that without the extinction cycles, synthetic intelligence would completely annihilate all organic life. This is because of a natural tendency to chaos; organics will always create synthetics and synthetics will always rebel against them. The Reapers also believe that destroying advanced civilizations is the only way to allow new civilizations to come into existence. The Reapers leave no evidence of their conquest, nor 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 an 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 explains, "By using it [mass relay technology] your society develops along the paths we desire."

Once the sentient races have established themselves on the Citadel with the aid of the keepers, a lone Reaper vanguard stationed within the galaxy 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, harvesting their populations or enslaving them through indoctrination.

The extinction events may be part of the Reapers' reproductive cycle, in which all the sentient life in the galaxy is harvested and essentially melted down into techno-organic Reaper shells based on the individual species' physical form, as seen with the Human-Reaper in Mass Effect 2. This is further expanded upon in Mass Effect 3 where it is explained that the Reapers harvest organic civilizations at the height of their power in order to "preserve" the individual species in Reaper form.

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 first living Reaper to be witnessed by any living intelligent being after the Protheans became extinct was designated Sovereign by the organic races (although the geth knew it by the name "Nazara"). 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.

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 realize the connection between the Reapers, the Citadel, and the keepers, but this discovery gave them the key to breaking the cycle forever and they set about attempting to preserve the sentient races they saw evolving on other planets, including the hanar and humans.

The Protheans developed a plan to forestall the impending Reaper attack for future generations of sapient, spacefaring species. This plan hinged on the fact that the keepers have independently 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. This agent was Saren, who became capable of comprehending 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 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.

The Reapers and the Collectors
After Commander Shepard defeated Saren and Sovereign, the Collectors began attacking human colonies and abducting their populations. Cerberus determined that the Reapers were behind this and planned to have Shepard thwart this latest Reaper threat to humanity. Over time, Shepard uncovered disturbing facts regarding the nature of the Reaper connection to the Collectors. Apparently, because humanity is a race of great genetic diversity and was the race who defeated Sovereign, it was enough to gather the Reapers' attention. It is revealed that the Collectors were originally Protheans who were captured by the Reapers and genetically re-purposed to suit their needs.

The Collectors were working under the direct supervision of the Reaper Harbinger, who ordered the Collectors to abduct humans in the Terminus Systems. The captured humans were taken to the Collector Base and processed into organic matter to construct a new Reaper modeled on the human form. EDI speculated that this was the Reaper equivalent of reproduction. When discovered by Shepard, the incomplete Human-Reaper was composed of facsimiles of the skull, arms and ribcage of a human with its lower spine and torso still under construction. EDI concluded that tens of thousands of humans had already been processed. Shepard was able to stop the process and destroy the Human-Reaper.

With the Human-Reaper destroyed and the Collectors defeated, the Reapers lost any chance of using the Citadel mass relay to quickly return to the Milky Way Galaxy. Harbinger and the rest of the Reapers, who have already been on the move since the destruction of Sovereign, are shown approaching the Milky Way, setting the stage for Mass Effect 3.

Arrival
As the Reaper fleet draws closer towards the galaxy, Dr. Amanda Kenson was exposed to a Reaper artifact showing her visions of the Reapers' plan: to enter the galaxy using the mass relay in the Bahak System, which sits at the edge of the galaxy. In order to delay their arrival, Kenson decides to destroy the relay by ramming an asteroid into it. However, before the plan could go to effect, Kenson and her team became indoctrinated by the artifact and she and a small team were then later captured by batarians.

This course of events would prompt Admiral Hackett to send Commander Shepard out on a solo mission to rescue Dr. Kenson and see what she was up to. Despite resistance from the indoctrinated Kenson and team, Shepard was able to complete the mission and destroy the Alpha Relay at the cost of three hundred thousand batarians living on Aratoht.

Return to the Galaxy
In Mass Effect 3, the Reapers finally return from dark space, thus beginning their next galaxy-wide harvest of advanced organic civilizations. Their invasion begins in batarian space and rapidly spreads throughout the galaxy.

Purpose
A Prothean VI named Vendetta on Thessia explains that the patterns concerning the development of galactic civilization and the cycles of extinction are too regular and repetitive to be coincidence. According to Vendetta, the galaxy experiences the same paths and conflicts during each cycle - "the same peaks of evolution, the same valleys of dissolution" - expressed in different forms. It infers that the Reapers are servants of this pattern, not the creators of it; however, it can't identify their true master or motive.

According to the Catalyst, the Reapers were created for the sole purpose of ensuring the ongoing existence of organic life in the galaxy, based on the assumption that all synthetic intelligences will eventually destroy their organic creators. This end result is believed by the Catalyst - and the Reapers - to be inevitable, and indeed available evidence seems to point to the validity of their assertions: the protheans' civilization was threatened by synthetic uprisings before the Reapers wiped them out, and the geth of modern times are viewed as a substantial threat to peace in the galaxy.

By harvesting technologically advanced species before they are capable of creating truly intelligent synthetic life, and then storing these old species within immortal Reaper bodies, room is made for new life to flourish and grow, as was the case for primitive man. The continuity of life in the galaxy is assured through this cycle of extinction, as it ensures that organic life will never be fully exterminated before its time by synthetic life, as was demonstrated by the quarians and the geth.

Fate
During the Battle for Earth, depending on Shepard's decisions and the level of galactic readiness achieved, the Catalyst gives Shepard three options:
 * Destroy the Reapers (Red) - Shepard will shoot and destroy a power conduit, causing the Crucible to unleash an energy pulse which will destroy the Reapers, the geth, and all other forms of synthetic intelligence in the galaxy;
 * Control the Reapers (Blue) - Shepard will transfer their consciousness into the Crucible, destroying Shepard's physical body but allowing them to gain control over the Reapers;
 * Synthesis (Green) - Shepard will leap into the Crucible's energy beam, and the Crucible will emit an energy pulse that converts all life in the galaxy on the molecular level, causing all organic life to have partially synthetic traits and vice versa.

If Shepard chooses the first option, the Reapers will shut down and collapse. If the level of galactic readiness is below a certain level, the Crucible will misfire, causing the energy wave to kill both synthetic and organic life. If the level of galactic readiness is high enough, however, the Crucible fires properly and Shepard manages to survive the event.

In the latter two options, the Reapers cease their attacks and leave the galaxy peacefully, albeit Shepard is unavoidably killed. All endings have the Mass Relays being destroyed or disabled to spread the energy from the Crucible across the galaxy.

Notable Reapers

 * Derelict Reaper
 * Harbinger
 * Human-Reaper
 * Leviathan of Dis
 * Sovereign

Trivia

 * The idea of a terrifying and incomprehensible alien intelligence waiting in the depths of space is a feature of Lovecraftian horror. This similarity is emphasized by one of the recordings by the survey team aboard the derelict Reaper which spoke of how "even dead gods can dream." This is a clear homage to Cthulhu, as "In his house at R'lyeh dead Cthulhu waits dreaming."
 * Another link between the Reapers and Lovecraft's work is his philosophy on Cosmicism. The Reapers are portrayed as incomprehensible with reasoning and long-term goals that are made out to be impossible for organics to ever understand. They also claim to be the pinnacle of existence and that organic beings could never obtain this level of meaning on their own.
 * From the cinematic showing the Reaper fleet at the end of Mass Effect 2, there are roughly 295 Reapers visible (as seen in this screenshot).
 * According to Legion in Mass Effect 2, a Reaper consists of "billions of organic minds, uploaded and conjoined within immortal machine bodies."
 * Dialogue was removed from Mass Effect 2 that details the Reaper harvesting process. EDI states that the captive humans were being reduced to their basic components by being dissected down to the atomic level. The data from the process could then be uploaded into a Reaper's neural network, thus storing the knowledge and essence of the individual that was liquefied in the process. Harbinger indicates that being turned into a Reaper is a form of rebirth. In reference to the fight with the Human-Reaper, Harbinger also states that Shepard is the one "wasting lives."