Collector

The Collectors are an enigmatic race that live beyond the Omega 4 mass relay, within the same system as Omega, within the Terminus Systems. They are rarely seen in the Terminus itself, let alone Citadel space, and are regarded as a myth by Citadel citizens. Definite sightings of Collectors have been made on Omega every few centuries. They are thought to be highly advanced and are known for their odd trade requests.

The Collectors offer new technologies, often of a startling level of advancement, in return for -- for example -- two dozen left-handed salarians, sixteen sets of batarian twins, a krogan born of parents from feuding clans, or two dozen "pure" quarians (quarians that have never left the Migrant Fleet due to illness, importance to the fleet, or disability). One of their current interests is in healthy human biotics. No one knows what happens to the individuals concerned after the exchange is completed.

Culture
The Collectors usually operate through agents, such as the quarian Golo, to make their deals. It has been speculated that the Collectors make these strange requests because of strange fetishes or gruesome culinary tastes. However, it is more likely that they perform genetic experiments on these subjects. Regardless of the possible danger, their requests are often granted by fringe groups because the technology the Collectors offer in return is often extremely valuable, giving any race who receives it an immediate advantage before it becomes adapted to the galactic community.

All known expeditions to investigate the Collectors have failed. No vessel that has ventured beyond the Omega-4 relay has ever returned. Some speculate that there is a black hole at the relay's counterpart, but this raises the question of how the Collectors can safely use it. Another, rather fantastic, explanation is that on the other side of the relay is a paradise which no citizen of Omega would care to leave due to the crime and suffering of the asteroid. A more likely reason is that the Collectors either detain or destroy any vessels encroaching on their space.

The Collectors are controlled by a single leader who can possess any member of its species at any time, instantly turning them into the most dangerous Collector on the battlefield. When a Collector is possessed by the leader the outer surface of their body appears to crack open, revealing an orange glow that resembles molten rock.

Biology
Collectors are insect like and have fully developed wings that allow them to traverse the battlefield. Their skin is susceptible to modern weaponry, including explosives, incendiary rounds, and cryogenic rounds. Periodically, As explained above, Collectors have been seen to be 'possessed' by an overseer at which point their skin begins to emit a bright light. At this point they become more resilient and are capable of discharging powerful missiles from their body.

Mass Effect: Ascension
The quarian Golo was a member of the Usela's crew, until he made a deal with the Collectors. They wanted two dozen 'pure' quarians from the Flotilla, quarians who were uncontaminated because they had not gone on their Pilgrimage. Golo agreed to sell his own people in exchange for new technology, but he was discovered and punished with exile.

In 2183, Golo offered Cerberus operative Pel a deal to give healthy human biotics to the Collectors, in exchange for enough credits to escape Cerberus. Thus, the latter planned on giving the captured Gillian Grayson and Hendel Mitra in, as well as throw in Kahlee Sanders, a specialists on biotics and Paul Grayson in as bonuses. However, they were rescued by Lemm'Shal nar Tesleya before the deal could be transacted and Pel and his cohorts were all killed.

Mass Effect 2
Shortly after the defeat of Sovereign, the SSV Normandy is attacked by a Collector Cruiser and destroyed. Most of the crew is able to escape though Commander Shepard and 20 crew members got killed including Navigator Presley (according to Mass Effect 2 DLC:Normandy Crash Site). Two years later and the Collectors, always believed to be a fringe threat, have begun abducting the populations of human colonies. Cerberus is intent on stopping these abductions, especially after realising that there is a Reaper connection to these Collector attacks, and so the group revives and recruits Shepard to investigate the Collector threat.

The Collectors attack human colonies by arriving in their enormous ship and releasing Seeker swarms, insect-like devices that paralyze everyone in the colony. Once this is completed the Collectors gather up all the paralyzed people, put them in stasis pods, and leave. Due to the Reapers being behind these Collector attacks, specifically Harbinger, the Collectors are also using Husks.

While exploring an apparently derelict Collector ship, it is discovered that the Collectors are genetically rewritten Protheans, assumably under Reaper indoctrination. After extreme genetic tampering the Collectors are almost completely different from Protheans, sharing only a single genetic trait which happens to be unique to them both (as pointed out by EDI).

After Shepard obtains a means to safely travel through the Omega 4 Relay to what is thought to be the location of the Collector homeworld in the galactic core, Shepard's team instead finds a space station: the Collector base. After overcoming an extensive debris field, Oculus defences and destroying a Collector Cruiser, Shepard and his team infiltrate the Collector base where they discover that the Collectors have been processing the tens of thousands of humans they have captured, transforming them into a genetic paste and using it to create a Human-Reaper. Shepard eventually destroyed the Human-Reaper and proceeded to set a countdown which would destroy the Collectors on the base, possibly wiping out the entire species. As the Collector General perished, he was abandoned by Harbinger.

Trivia

 * The actions of the Collectors vaguely recall the mythological sacrifice of Athens, where seven young men and women would be sent to the island of Crete every year as tribute, only to be devoured by the Minotaur deep in the Labyrinth.
 * This could have been partially influenced by the "To Serve Man" episode of the Twilight Zone, in which an alien race provided much useful technology before they invited many humans to their planet to be "served". The humans believed they were being brought to a paradise planet when in reality they were to be cooked and eaten by the aliens ("To Serve Man" being the name of the aliens' cook book). [1]
 * The leader of the Collectors bears a resemblance to "Pilot" of the Farscape series.
 * The Collectors were originally inspired by electron microscope photographs of insect heads. Their full-body designs ranged from humanoid to highly-abstract creations.