Reapers (Mass Effect)

The Reapers are an ancient race of gigantic living spaceships and the main villains of the Mass Effect trilogy. Initially thought to be a myth by most of the characters, the reapers hibernate in "Dark space" for thousands of years and then return to the galaxy every few millennia to destroy all the Milky Way's most advanced civilizations, most notably the "Protheans", who gave them the name "Reaper". Their motives for doing this are completely unknown, and they claim that it is beyond mortal understanding. The origin of the reapers is a mystery too, and it is said that they have existed since the beginning of time.

Reapers have the ability to gradually brainwash anyone who comes into prolonged contact with them, a process known as "Indoctrination", and also to directly take complete control of an organism's body. It is also revealed that they were the ones who originally built the Citadel, the main base of galactic civilization, intended as a "trap" of sorts to attract a large number of advanced life forms together in one place.

So far, only four reapers have been encountered in the series:
 * Sovereign: The main antagonist of the original Mass Effect (besides Saren Arterius). Sovereign initially appears as Saren's flagship, but is later revealed to be the leader of the reapers who would act as the "Vanguard" initiating and leading their eventual return to the galaxy by activating the mass relay at the Citadel for the other reapers to enter through. However, Commander Shepard killed Saren and installed a defense program in the Citadel's mainframe, delaying Sovereign and allowing him to be destroyed by the Citadel fleet, preventing the return of the other reapers for the time being.
 * Harbinger: The main antagonist of Mass Effect 2 and the successor of Sovereign. He was the controlling force behind the "Collectors", who were actually warped versions of the Protheans. He directly possesses the Collector General most of the time, and can also take control of any normal Collector at any time to fight Shepard. At the end of the game, he relinquishes the Collectors and is seen in his true reaper form leading the other reapers as they slowly travel towards Earth.
 * Derelict Reaper: A "dead" reaper floating in space visited in Mass Effect 2, which holds vital research. Here, a friendly Geth named "Legion" is met. At the end of the mission, Shepard battles the reaper's still-intact core protected by an army of Husks, after which the reaper is destroyed completely.
 * "Human-Reaper": The final boss of Mass Effect 2, this monstrosity was a new, smaller, humanoid version of the Reaper being made by the Collectors led by Harbinger out of processed human DNA. It was discovered in the center of the Collectors' base by Shepard and friends, and destroyed before it could be completed.

In addition, hundreds of currently unidentified reapers are seen along with Harbinger in the ending sequence of Mass Effect 2 flying towards the inhabited Milky Way, and in the trailer for the upcoming Mass Effect 3, several reapers are seen laying siege to Earth. It is unknown how the player will be able to fight them in the game.