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.
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 advanced civilizations 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. The purpose of these eradications is two-fold:
- 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 the form of the race that supplied the genetic material, and an insectoid shell is put over it. The harvest of advanced races preserves their knowledge, culture, and biological characteristics for the future, making way for less advanced species to grow into interstellar civilizations.
- The harvest prevents galactic war between organic races and synthetic races. The Reapers always target races that have begun to develop artificial intelligence technology. The Reapers believe that, left unchecked, such organic-synthetic warfare would inevitably result in the destruction of all organic life.
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. It 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, which would be able to summon 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 mobilized and headed 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 relay on the edge of the galaxy, known as the "Alpha Relay", which lay in batarian space. 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 Aratoht star system as a result. Without a relay to quickly leave the star cluster, the Reapers were forced to continue deploying via regular FTL, delaying their invasion of the galaxy by months. Nonetheless, the Reapers soon arrived in force and began harvesting the more advanced races of the galaxy.
Each Reaper is a sapient life form controlled by a gestalt artificial intelligence. They are created by melting down millions of selected specimens from an organic species and using their DNA material to embody the species in the Reaper.
A Reaper is actually a gigantic cyborg. A synthetic shell modeled after the first Reaper is constructed around the organic core. The shell is armored and protected by powerful kinetic barriers. A Reaper is armed with one or more main guns that fire streams of molten iron-uranium-tungsten alloy accelerated to a significant fraction of c, with a range measured at tens of thousands of kilometers. They also mount laser defenses for shooting down enemy fighters and missiles.
An especially dangerous capability of the Reapers is indoctrination. Reapers produce an energy field that slowly alters the limbic systems of the brains of sapient species. Prolonged exposure to the field will cause those exposed to be controlled by the Reapers unquestioningly. However, the more control a Reaper exerts over its victims, the less competent the victim becomes. Some devices created by Reapers also generate this field, and the body of a dead Reaper can still indoctrinate beings in its vicinity.
There are two major Reaper variants.
Reaper Capital Ship
A Reaper capital ship (or "Sovereign-class" Reaper) is a two-kilometer-long warship and by far the heavier class of Reaper. A Reaper capital ship's main gun has an estimated firepower between 132 to 454 kilotons (presumed to be peak output per shot). A Reaper capital ship's kinetic barriers can withstand the sustained firepower of two Citadel-race dreadnoughts (about 76 kilotons every 2 seconds) indefinitely, while the sustained fire from three dreadnoughts causes it to show strain, and four dreadnoughts are enough to inflict damage. This suggests that the Reapers have a high recharge rate of their kinetic barriers relative to their full strength.
A Reaper destroyer is a much smaller warship, but it still measures hundreds of meters in length. They are made from the "lesser" races harvested during a cycle, and they have more design similarities than the highly individualized Reaper dreadnoughts.
Other ship types
The Reapers use several non-sapient ship types which are remotely controlled by sapient Reapers. These include troop transports, which vary between 200 meters and 1 kilometer in length, and processor ships which are used to harvest DNA material from captured populations.
Reapers of Note
- Sovereign (Nazara): The vanguard left behind after the last Reaper invasion to initiate the next one at an appropriate time.
- Harbinger: The oldest and possibly the most powerful of the Reapers.
Reapers are highly advanced technologically compared to other races in Mass Effect. Their kinetic barriers are more powerful than those of other Mass Effect races, and they also seem to have a high recharge rate. Their weapons are also an order of magnitude more powerful than those of the other races. It takes at least four turian dreadnoughts to have a chance of significantly damaging a Reaper, and there are hundreds, possibly thousands, of Reapers. Creating a new Reaper is, however, a prolonged process -- essentially, a whole organic species must be harvested and embodied in a Reaper.
Reaper technology allows them to manipulate the biology of other species, creating armies of mutated slaves collectively known as husks. The abilities of husks vary depending on the species used to create them. Husks can be rapidly produced in large numbers wherever Reapers have established a presence.
Reapers do not seem to require supply lines. They typically destroy fuel and ammunition depots in systems where they gain control, and they only use transports for moving husks from one theatre of operations to another.
The Reapers could be considered a significant threat to the United Federation of Planets, but not nearly as much of one as to the Mass Effect races. The Federation doesn't rely on mass effect technology, meaning that Reaper weapons and strategies are not optimized for fighting them.
They probably would not be very threatening to the Galactic Empire.