The Leviathans are an ancient aquatic race that dominated the galaxy in the time before the Reapers. Proud and aloof, they regard themselves as the galaxy's first and only apex race, and all other races are merely tools meant to serve them. With the advent of the Reapers, the Leviathans were forced into hiding to preserve themselves, and were content to observe the Reapers' harvests while surreptitiously erasing any evidence of their existence over the course of hundreds of millions of years.
Like the Reapers that were designed after them, the Leviathans are massive in size and possess a resemblance to Earth cuttlefish or squid. Two sets of three eyes are embedded symmetrically on the lower dorsal surface of the body. Several tentacle-like appendages extend below the body and from the sides. The Leviathans' exterior is covered in bony, heavily ridged plates. As Leviathans are aquatic, it is believed that they can breathe oxygenated water and their bodies can withstand both the extreme pressure of ocean depths and the lesser pressure of coastal areas.
The Leviathans possess the natural ability to influence the behaviors of lesser-minded organisms to the point of complete physical and mental control, an effect that is similar to indoctrination. They can use this ability to communicate with other beings in a manner similar to telepathy, as well as to turn them into thralls. The exact mechanics of this ability are unknown, but it has been compared to the rachni's theorized ability to communicate using an organic quantum entanglement communicator analogue that stimulates neural activity. Over-exertion of this mind-control ability causes nosebleeds in humans, and in extreme cases can lead to brain damage and death.
It is possible that the Leviathans further enhanced themselves cybernetically, allowing them to use element zero to travel through space and breathe air with the use of filtration systems. They might even have modified themselves with biotic amplifiers. In addition, their faster-than-light communication pulses require precise control with technology completely unknown to modern science.
The early history of the Leviathans is completely unknown, but it is speculated by Task Force Aurora that they evolved on an unknown planet that also produced at least one other sapient species. The Leviathans developed the ability to communicate with and dominate land-based sapient species to serve their purposes. Eventually, the Leviathans utilized this thrall species' civilization to achieve spaceflight and spread throughout the galaxy, despite their immense size and aquatic nature. Every new species they encountered was enthralled to serve them and provide "tribute", and in return, the thralls were cared for and protected. With their dominance assured, Leviathans considered themselves the galaxy's apex race. Over time, however, they observed that their thralls would frequently build synthetic intelligences to aid them; these synthetics consistently rebelled, wiping out many thrall species. In response, the Leviathans created an Intelligence with the mandate to preserve life at all costs.
The Leviathans failed to perceive the Intelligence as a threat; they considered it nothing more than another tool to achieve a set goal. As the Intelligence sought out the means to fulfill its mandate, it created an army of pawns that were dispatched to collect genetic data from species throughout the galaxy, much like the Collectors employed by the Reapers in modern times. Eventually, the Intelligence came to the conclusion that the Leviathans themselves were part of the problem, and turned against them.
The Intelligence's betrayal of its creators was sudden and devastating. Its pawns slaughtered most of the Leviathans, using their genetic material to create the very first Reaper, Harbinger, who was physically designed after the Leviathans themselves. The surviving Leviathans went into hiding, while the Intelligence used Harbinger to begin the cyclical harvest of the galaxy's species to create more Reapers, all in an effort to solve the problem of preserving life. While the Leviathans acknowledged their creation's betrayal and did not wish to be driven to extinction, they believed the Intelligence was still fulfilling its purpose.
As the Leviathans hid, they surreptitiously enthralled subsequent species to erase the remaining evidence of their existence. Resigned to the Reapers' reign, they focused their efforts on ensuring the survival of their species, watching the galaxy through organic "artifacts" for signs of their discovery. Though they had the power to destroy Reapers, demonstrated when they killed the Leviathan of Dis, the Leviathans declined to aid the galaxy's species against the Reapers.
During the Reaper invasion of 2186, a Systems Alliance group known as Task Force Aurora began to threaten the Leviathans with discovery. Dr. Garret Bryson, the head of the task force, took interest in the Leviathan of Dis and surmised that whatever killed it might be an advantage against the Reapers in the present war. Under orders from Admiral Hackett, Aurora began investigating incidents and patterns that might lead to this Reaper-killer, the "true Leviathan of Dis". Bryson pursued several leads, including sightings of mysterious creatures around the galaxy, sites that might be frequented to gather element zero for space travel, and the activity of the Reapers as they seemingly carried out their own hunt for Leviathan. Aurora members Dr. Alex Garneau and Dr. Ann Bryson were dispatched to nexuses of these data sets, Mahavid and Namakli, to search for artifacts related to Leviathan and pick up its trail.
At Admiral Hackett's request, Commander Shepard meets with Dr. Garret Bryson to assist in his research. When Bryson is murdered by his assistant, who was being controlled by a Leviathan through an artifact in Bryson's lab, Shepard continues the scientist's work. By this time, the Reapers had followed in Aurora's footsteps and sent their own forces to Mahavid and Namakli to seek the Leviathans. A breakthrough occurs when Shepard rescues Ann Bryson, who had been enthralled by a Leviathan artifact found on Namakli. Using the artifact in Aurora's lab, Shepard allows Ann to be briefly enthralled and traces the Leviathan's communication signals to the remote 2181 Despoina.
There, Shepard dives to the Leviathans' lair deep under the planet's ocean and meets a Leviathan, which uses its powers to trap Shepard in their own mind. Projecting itself as Derek Hadley, a man posing as Alex Garneau, and Ann Bryson, Leviathan explains its race's history and connection to the Reapers. Initially, Leviathan refuses to help Shepard, preferring to keep the Commander prisoner while the Reapers harvest the galaxy as they have always done, but Shepard points out that the Reapers now know of their existence and that the Leviathans can no longer afford to remain in isolation.
Realizing that the Reapers fear Shepard for defeating Sovereign and the Collectors, other Leviathans reveal themselves. Together, they agree to aid the galaxy in the war against the Reapers because they still consider themselves the galaxy's apex race, not out of a desire to help "lesser species". As Shepard departs the planet, the Leviathans turn a Brute against other husks and use their artifacts to transmit a powerful pulse that disables an attacking Reaper Capital Ship, illustrating their power.
Virtually nothing was known of the Leviathans before Commander Shepard's encounter with them. No record of the Leviathans' true name exists, no ruins are attributed to their civilization, and no influence on other races has been recorded. The only traces of their existence known to remain are ancient rock paintings depicting them on Namakli, and numerous mysterious "artifacts" scattered across the galaxy.
Technologically, the Leviathans claim and are believed to be extremely advanced, even more so than the Reapers. They have the capacity to direct faster-than-light communication pulses via a method unknown to modern science and created many organic, opalescent "artifacts" through which they can communicate, control thralls, and observe the galaxy. The Leviathans' ability to develop and use organic tools has been compared to the Thorian, which similarly evolved to eschew the conventional technology depended on by other sapient species.
The Leviathan Enthrallment Team is a mixed-species team of special-operations soldiers that have volunteered to covertly deliver Leviathan artefacts behind enemy lines. These attacks aim to make Reaper creatures thralls of the Leviathans. On a small scale, this could cause chaos and disruption. On a large scale--and if given enough time--it could form an army of Reaper creatures dedicated to exterminating their own kind. The team's major limiting factors are the small number of artefacts and the reluctance of Alliance commanders to operate openly with such a potent weapon for fear the Reapers might develop a countermeasure.