The Mass Effect trilogy has become well-loved for its engaging characters, its RPG mechanics and customization options, and its branching storylines affected by player decisions. But it is also known for its main antagonists. In the first game, Shepard was pursuing what appeared to be a rogue spectre named Saren who had mysteriously obtained an impressive ship to use for his own ends - but in a twist, it was the ship that was in charge. That ship, Sovereign, was Shepard's first encounter with a reaper - a dangerous machine actively planning the destruction of every advanced civilization in the galaxy.
And after barely defeating Sovereign, it turned that there were others of its kind - the Reapers, waiting just outside the galaxy's edge for their moment to strike. The Reapers were an entire race of sentient spaceships controlled by artificial intelligence, constructed at a technological level that far surpassed anything the Alliance had at their disposal. There were potentially millions of them and they had one goal - to wipe out every spacefaring civilization by any means necessary.
7 Nearly Indestructible
One of the biggest problems fighting the reapers is that they are extremely durable, at least against anything the alliance can throw at them. In the first game, it took their entire combined military force just to take down one Reaper, and even that was at a high cost only accomplished because Shepard found its one weakness (a weakness that the reapers learned from and made sure could not be exploited again). By the third game, there is an entire army of these ships. Their massive size and heavy weapons make it easy for them to cause massive devastation before the enemy even has time to react.
6 Created Mass Relays
Though it was originally assumed that the mass relays used throughout the trilogy were made by the Protheans, they were actually a strategic tool used by the reapers. The relays served multiple functions. The most obvious is that they were carefully placed in locations where they were likely to be discovered and therefore exploited by new spacefaring civilizations. This actually served as a means of control, the idea being to ensure that every race followed a similar evolutionary path that could be anticipated and planned for by the reapers. It also acted as transport for the reapers themselves, allowing them to more efficiently conduct their harvest at the end of each cycle.
5 Cycles
The Reapers oversee what they view as an unbreakable cycle revolving around the rise and fall of spacefaring civilizations at specific intervals. Over the course of 50,000 years, at least one species makes the transition from stone age tribes to a spacefaring civilization. That civilization is allowed to build itself and thrive until it reaches its peak, which is considered the end of the cycle. At this point, the Reapers enter and begin a campaign to wipe out the currently dominant races.
As counter-intuitive as it may sound to let civilizations rise only to destroy them, the Reapers consider it a way of maintaining order. Their reasoning is that maintaining cycles ensures that different races have their time in the spotlight.
4 Origins
It is unknown exactly where the Reapers were first created, but their origins go back thousands of years to a forgotten species known only as "Leviathan."
Leviathan was once the dominant power in the galaxy and ran a massive empire that enslaved other races. In order to assert its dominance, Leviathan would demand what it considered "tribute" from its slaves, though exactly what would have been given to them is unknown. However, they found a problem - over and over again, subjugated species would try to build artificially intelligent machines to make work more efficient only to see them rebel. The resulting conflicts between organic and synthetic life would often have disastrous results and end in massive casualties, if not entire extinctions. Leviathan quickly determined this was a problem - after all, an extinct race cannot provide tribute.
The solution was to design an AI system programmed to find a way to preserve life in the galaxy. Unfortunately, the computer determined that Leviathan itself was part of the problem and that it was impossible for organic and synthetic life forms to coexist- so it created a system where a cap was put on how long an intelligent civilization was allowed to last before it was destroyed with a new one being allowed to take its place. That led to the first harvest, against Leviathan, and also the first reaper, Harbinger.
3 Harvesting
At the end of each 50,000 year cycle, the Reapers step in to wipe out every intelligent civilization in the galaxy, before retreating into space and allowing a new one to rise. Those who are not exterminated are harvested, a process that involves mass kidnapping of thousands of individuals of a particular species and breaking down their bodies. The victims are used as components to build a new reaper, one in their race's image. This acts as a means of reproduction, with each new reaper being made of and resembling the dominant race of its birth cycle, but they also see turning other races into reapers as a way to preserve them.
2 Soldiers
Reapers do not work alone, they have support from an organized military force that - in a cruel twist of irony - is made from the very species they have been harvesting. Those unfortunate enough to go be dragged into the Reaper forces have their free will and individuality stripped from them, and their bodies transformed to better fulfill their new role as a slave. The end result often leaves the victims barely recognizable, leaving little more than the general shape of whatever species they once were.
Different races are often turned into different types of soldiers. Humans are usually transformed into "husks" - basically zombies that can swarm enemies in large numbers. Meanwhile, Turians are transformed into "Marauders" that can be deployed to act as squad leaders, and Asari are converted into "Banshees" that can use powerful biotic attacks; and Batarians are transformed into "cannibals" that act as elite soldiers who can quickly recover from heavy damage. Sometimes they even combine the most useful qualities of different races, such as the brute - a hybrid of Krogan and Turian physiology that acts as a tank for the Reaper forces.
The most extreme example is the collectors. These were originally remnants from the Protheans, before the Reapers turned them into a hive-minded slave race. Through thousands of years of careful genetic manipulation, the Protheans lost all capacity for independent thought but still retained the intelligence needed to use reaper technology and coordinate on the battlefield.
1 Indoctrination
One of the most terrifying weapons in the Reapers' arsenal is the ability to manipulate individuals of other races through a process known as indoctrination. The full process is unknown, but it involves using an electromagnetic field to stimulate certain parts of the brain in order to influence the actions of their victim, with a barely audible voice telling them how to act.
The victim is turned into a slave of the Reapers, who will use them to further their own ends. Indoctrination is also a subtle process, which infects the victim gradually without their knowledge. This can be used to warp the victim's mind and allow them to think they are acting on their own accord, or for the benefit of the galaxy. Saren was tricked into thinking he was offering the one hope for humanity's survival, the Illusive Man was tricked into thinking he was offering a better solution - both were being controlled. The Reapers can even warp a person's mind to think they're not being indoctrinated and acting entirely on their own initiative.
The worst part is that once a person is indoctrinated, there is no escaping. Trying to resist the impulses brought on by indoctrination only causes extreme pain, so at most the Reapers' link can only be broken for a few minutes. No cure exists. The Reapers will never let go of an indoctrinated servant. And as it progresses, the indoctrinated victim experiences severe brain damage. Indoctrination only stops when the victim is either dead, or their brain has deteriorated to the point where they might as well be.