Reaper
Warning: this page contains major spoilers for the Mass Effect video game series.
The Reapers are a race of sentient starships from Mass Effect. They are responsible for the construction of the Citadel space station and the mass relays integral to transit in the Mass Effect galaxy. Reapers are generally two kilometers long, and their consciousnesses are each composed of thousands of AI programs.
History
The Reapers normally hibernate outside of the galaxy in dark space, for tens of thousands of years at a time. Every time a spacefaring race reaches a high enough technology level, the Reapers will return to the galaxy through the giant mass relay that is the Citadel to wipe out all intelligent life in the galaxy. This cycle has gone on for at least 37 million years. The mass relays are set up to more easily facilitate this- each civilization inevitably becomes dependent on the relays, and since the Citadel sits at the center of the relay network, it usually becomes the center of the galactic government. When the Reapers return through the Citadel, they quickly take out that government and shut off access to the relay network, throwing the galaxy into disarray. From that point, each civilization is easy to defeat, though total eradication can still take decades. Apparently, the purpose of these eradications is reproduction. The Reapers must harvest genetic material from a species with enough genetic diversity in order to produce a new Reaper. The new Reaper takes the internal form of the race of its genetic material, until an insectoid shell is put over it.
After each genocide, a single Reaper is left as a vanguard, to monitor the development of new civilizations and signal the Citadel mass relay to be activated, bringing the whole Reaper fleet. The actual activation is done by the Keepers, a non-sentient race of arthropods that automatically operate and maintain the Citadel. The last race to be wiped out by the Reapers, the Protheans, managed to screw with this. After the Reapers returned to dark space, a handful of Prothean survivors returned to the Citadel and messed with the Keeper's programming. No longer would the Keepers open the Citadel at the signal of a Reaper vanguard- the vanguard would have to physically link up to the Citadel in order to open it. Thus, the next civilization would have a chance of survival the Protheans and all their predecessors never had. The last Reaper vanguard, Nazara (known as Sovereign to its followers) attempted and failed to open the Citadel, being destroyed in the process thanks to the efforts of the Systems Alliance operative Commander Shepard.
With Nazara's failure, a Reaper called Harbinger took a different approach. He used the Collectors, a race that was created by the Reapers from the DNA of the Protheans, to harvest humans and make a new Reaper without invasion by the Reapers. Once again, Shepard foiled this plan, destroying the Reaper-human "larva". After this second failure, the whole Reaper fleet has apparently awoken and is heading towards the Milky Way galaxy on regular FTL.
Threat Assessment
Reapers are highly advanced technologically compared to other races in Mass Effect. Their kinetic barriers are impervious to double-digit kiloton-level kinetic impacts. However, a single Reaper probably could not defeat the combined forces of all the Mass Effect races. There is only one case of a fully functional Reaper being destroyed. This was accomplished by a massive stationary mass accelerator cannon, built by a race probably hoping to strike one last blow against the Reapers. The minimal power required to destroy a Reaper, however, remains unknown, since the weapon was a significant overkill (even after passing through the ship, the round retained enough kinetic energy to gouge an enormous canyon thousands of kilometers long in a nearby planet, blasting apart millions of cubic kilometers of rock). The Reapers are armed with special mass accelerators that accelerate a liquid iron-uranium-tungsten alloy shaped into armor-piercing projectiles to a fraction of c, with the physical appearance of a beam. Despite their size, they are highly maneuverable, capable of executing turns which would rip apart any Alliance ship. There are hundreds, possibly thousands, of Reapers. They would be considered a major threat to the United Federation of Planets, but probably would not be very threatening to the Galactic Empire. A notable weakness is the long process it takes to make a new Reaper. Another weakness was that if a Reaper concentrated on remotely controlling a robotic body, the destruction of said body could cause a shutdown of the ship's systems, leaving it vulnerable to attack.
A notable ability of the Reapers is indoctrination. The Reapers produce a kind of energy field that slowly alters the limbic system in the brains of sentient species. Prolonged exposure to the field will cause those subjected to it to be controlled by the Reapers unquestioningly. However, the more control a Reaper exerts over its victims, the less competent the victim becomes.