Reaper
Warning: this page contains major spoilers for the Mass Effect video game series.
The Reapers are a race of sapient 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 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 process; growing civilizations inevitably become 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 genetically diverse species in order to produce a new Reaper. The "core" of the new Reaper takes form of the race that supplied the genetic material, and an insectoid shell is put over it.
After each genocide, a single Reaper remains behind 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 interrupt the cycle. After the Reapers returned to dark space, a handful of Prothean survivors returned to the Citadel and altered the Keepers' 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 that 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 the other Reapers from Dark Space. Once again, Shepard foiled this plan, destroying the Reaper-human "larva". After this second failure, the whole Reaper fleet is revealed to be awake and heading towards the Milky Way galaxy on regular FTL.
The Reapers had a back-up plan for overwhelming the galaxy -- if the vanguard failed to bring the Reaper fleet through the Citadel, the Reapers would head towards the galaxy using normal FTL methods. They would arrive at a special relay known as the "Alpha Relay", tucked away in an obscure part of the galaxy. From the Alpha Relay, the Reapers could quickly deploy themselves all over the galaxy, including to the Citadel. Yet again Shepard caught wind of this plan and aided in an effort to slam a giant asteroid into the Alpha Relay, destroying it before the Reapers could use it. The explosion from the relay's destruction wiped out the local star system as a result. Without a relay to quickly leave the star cluster, the Reapers will be forced to deploy using regular FTL, delaying their invasion of the galaxy.
Threat Assessment
Reapers are highly advanced technologically compared to other races in Mass Effect. They have two types of combat ships, two kilometer long capital ships and 160 meter destroyers. Their kinetic barriers are more powerful than other Mass Effect races, and also seem to have a high recharge rate. 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. There are hundreds, possibly thousands, of Reapers. Reapers also do not seem to rely on supply lines. 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 -- essentially, a whole organic species must be harvested and embodied in a Reaper.
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. The Reapers can also use nanites on captured enemies to control them and transform them into cybernetically enhanced forms.