In the universe of the Mass Effect video games, Reapers are a mysterious and malevolent race of sentient machines that exist in the dark space beyond the boundaries of the Milky Way. They are bio-mechanical starships of colossal size and power, but their existence is debatable as far as the galactic community is concerned. For countless millenia, they have engaged in a cycle of genocide by invading the galaxy and systematically annihilating all of the advanced organic races that have arisen over a 50,000 year period, harvesting whole populations as their 'solution' to the chaos of organic evolution.
Before the events of Mass Effect 3, absolutely nothing was known about the origins of the Reapers. In the Leviathan add-on content, it is revealed that the Reapers were originally conceived by a powerful race of aquatic-based creatures referred to as "Leviathans", who were once a dominant "apex" race that controlled all other races in the galaxy. However, a time came when the Leviathans' servant races created synthetic life to aid them with their various tasks, only to have their creations rebel. Seeing the struggle between organics and synthetics as a problem to be solved, the Leviathans created an "Intelligence" that would find a means of preserving life at all costs. Unfortunately, the Leviathans failed to realise that they had fallen into the same trap that their thralls had, by creating artificial life to solve a problem that organics could not solve alone. The Intelligence deemed that order could only be brought to chaos by harvesting all organic life, including that of its creators. And thus, the Leviathans were attacked and slaughtered by their own synthetic creations, their biological essence harvested to produce a new machine that existed to fulfill the Intelligence's mandate of preserving life. This machine was built in the image of the race that conceived it, and would create another in its likeness with every organic species it harvested. This monstrosity would become known in modern times as "Harbinger", the first of the Reapers.
For countless millenia, organic races have advanced to the point where they have created synthetic life, only for their creations to rebel against them. The Reapers' purpose is to allow organic societies to advance to a specific point, then harvest them before they create any artificial life that may threaten to destroy all organic life in the galaxy. All underdeveloped races are left alone and allowed to prosper until the next cycle while advanced cultures are harvested, the essence of all they were immortalised in Reaper form.
Once they have harvested all they need, the Reapers retreat once again to dark space and enter hibernation, awakening only when they recieve a recall signal from the Citadel: an enormous space station and mass relay created by the Reapers for the most advanced of organic cultures to find and use to establish galactic society. When the Reapers leave the Milky Way, they also leave behind a vanguard that observes the development of organics across the galaxy and reactivates the Citadel to signal the rest of the Reaper fleet to return when the time is right.
In the first Mass Effect game, the existence of the Reapers was brought to light when Human Systems Alliance officer Commander Shepard discovered a Prothean beacon on the colony world of Eden Prime, which was then under attack by Geth forces being commanded by Saren Arterius, a renegade Spectre agent for the Citadel Council. Saren's flagship Sovereign was in actuality a Reaper itself and had indoctrinated Saren into serving it and assisting with summoning the Reaper fleet from dark space. Sovereign and Saren sought out the Conduit - a Prothean-made mass relay that served as a "back door" to the Citadel, the hub of the mass relay network. With it, Sovereign's forces could infiltrate the Citadel and reactivate the signal that informed its Keepers to open the gateway to dark space, allowing the Reapers to invade and repeat their cycle of genocide. Both Sovereign and the crew of the SSV Normandy found the Conduit on Ilos, the site of a former Prothean colony and research facility. Assisted by the Prothean VI Vigil, Commander Shepard and his crew activated the Conduit and used it to travel back to the Citadel as it came under attack by Sovereign and the Geth fleet. Shepard and his team successfully defeated Saren before he could activate the gateway to dark space and Sovereign was destroyed by the combined Citadel/Alliance fleet. Despite this event, Reapers continued to be regarded as mythological by the galactic community and Shepard's claims were dismissed.
Mass Effect 2Edit
Although Sovereign had been destroyed, the Reapers were still able to maintain a presence in the Milky Way in the form of the Collectors: a race of cybernetic insectoids under the thrall of the Reaper known as Harbinger. The Collectors had previously been involved with illicit negotiations on the space station Omega, trading technology for rare genetic samples from various species. At the beginning of Mass Effect 2, a Collector ship attacked and destroyed the SSV Normandy as it was patrolling the Terminus Systems. Though many of the crew survived the attack, Commander Shepard was lost and presumed dead. Shepard's body was recovered by pro-human extremist group Cerberus, who ressurrected him using the most advanced medical and cybernetic technology available. Shepard was revived 2 years after the Normandy's destruction and was informed by Cerberus's leader - the Illusive Man - that the Reapers were behind several attacks on human colonies across the Terminus Systems. The colonies were left intact, but their populations had all disappeared without a trace. Cerberus provided Shepard with a new ship and the resources he would need to investigate the attacks, and sure enough he discovered the Collectors. Several engagements against the Collectors followed, including the abduction of the SSV Normandy-2's crew whilst Shepard and his commandos were off-ship. Shepard's team later discovered that the Collectors' base of operations was located in the galactic core, which could only be accessed via the Omega-4 mass relay. The Normandy-2 ventured into the relay and Shepard's team infiltrated the Collector base to rescue their crew and the abducted human colonists. There they discovered that the Collectors were processing human bodies and breaking them down into raw genetic material to use in constructing a Human-Reaper. The team rescued those they could and fought against the Human-Reaper larva whilst they tried to sabotage the base's systems. The larva was destroyed and the Collectors were exterminated. The Reapers were not finished yet, however, as Harbinger led the Reaper fleet as it descended upon the Milky Way.
Mass Effect 3Edit
The Reapers have arrived in the Milky Way and have launched a full-scale invasion. Their point of entry into the galaxy first brought them into conflict with the Batarian Hegemony, who had previously ignored all warnings regarding the Reapers and so were unprepared for the attack. The Batarians were conquered swiftly with only a handful of survivors managing to escape to the Citadel. When the Reapers attacked Earth, Shepard, formerly reprimanded for his involvement with Cerberus, was released from custody prior to the attack and manages to escape the planet with the help of his former commander, David Anderson. Anderson remains on Earth to rally a resistance against the invaders, while Shepard is assigned to gather allies and resources to help fight the Reapers. This leads to the discovery of an ancient Prothean device called the 'Crucible', supposedly a superweapon designed specifically to kill Reapers. Over the course of ME3, Shepard must gather resources and try to unite the galaxy's races, which is no easy task given the animosity between several of them. All the while, more Reapers appear across the galaxy, attacking planets and harvesting their populations (just as the Collectors did with the Humans they abducted). As the endgame nears, Shepard leads his forces against the Reapers occupying Earth. A long, brutal battle takes place in orbit while Shepard and crew land in London, England, where the Reaper presence is strongest. A Conduit has been established in London that links to the Citadel, which the Reapers have moved into Earth orbit for protection. At the very end, an injured Shepard boards the Citadel and fuses it with the Crucible. The Catalyst - an AI program that resides within the Citadel and the guiding intelligence behind the Reapers - leaves Shepard to make a final choice: he can use the Crucible to destroy the Reapers and end their menace to the galaxy forever, take control of them, or bring about a "synthesis" between organic and synthetic races, merging them into one and thus negating the Reapers' purpose. In each decision, Shepard sacrifices his life to activate the Crucible.
All Reapers are capable of dominating the minds of organic races through a process referred to as 'indoctrination'. Reapers and various technological items of their design emit some kind of signal or energy field that subtly influences a person's mind, compelling them to perform actions without knowing why. If proximity to a Reaper artifact is maintained over a period of time, the subjects simply stop thinking for themselves and just obey the Reapers' commands. The experience has been described as a compelling whisper that cannot be ignored or shut out, and even brief exposure can have a lasting effect that can interfere with a person's daily life. Indoctrination is a truly malicious and horrifying experience, however investigation into the indoctrination process by Saren Arterius has also revealed that there is a balance between control and usefulness. The more control the Reapers exert over their slaves, the less capable they become. This is evidenced by captured Salarian agents subjected to indoctrination, who have either gone mad or completely brain dead. The damage is severe and permanent, and the Reapers regard their slaves as entirely disposable, having abandoned all their slaves after every cycle of genocide and leaving them to die of starvation or exposure.
Reaper technology is highly advanced and makes them a virtually unstoppable menace, even without their powers of indoctrination. Each of a Reaper's feelers is a "magnetohydrodynamic" weapon that expels a stream of molten hot metal at a fraction of the speed of light, capable of obliterating a cruiser in one shot. They also carry immense guns mounted on their spines that can pulverize a dreadnought-class ships defenses with ease. With their enormous element-zero power cores, Reapers can generate powerful shield barriers and mass effect fields that will allow them to land on a planet. When landing on planets, Reapers can create their own ground forces in the form of Husks: cybernetic, zombie-like creatures that savagely attack anyone in sight. Husks are created by exposing living or dead organic bodies to devices called "Dragon's Teeth", which impale victims and infuse their bodies with invasive nanotechnology that remakes them into soulless killing machines.