"Leviathan" is a name given by humans to an ancient race of aquatic-based creatures that appear in Mass Effect 3's add-on content, Leviathan. These beings appear to be giant crustaceans and claim to be an "apex" race, a species that has reached the pinnacle of its evolution. Their intellect is vast and they possess the ability to completely dominate the minds of lesser species. Most importantly of all, the Leviathans are responsible for the creation of the Reapers.
Very little is known of Leviathan history. They were once the dominant race in the galaxy, several eons ago. They dominated lesser races to serve their purposes and in return protected them and enhanced them. However, the Leviathans' various thrall races eventually developed synthetic intelligence to aid them in better serving their aquatic masters, but the machines were not subject to the Leviathans' powers of dominance. As the synthetics advanced, they eventually rebelled against their makers and threatened the existence of all organic life. Unable to solve this dilemma on their own, the Leviathans created an Intelligence - which would become known as The Catalyst in modern times - with the mandate to preserve life at any cost. To fulfill its mandate, the Intelligence created numerous bio-synthetic pawns to collect genetic data from various species across the galaxy.
The Leviathans did not consider that the Intelligence they had created could be a threat, seeing it as merely another tool fulfilling the purpose it was designed for. It was only when the Intelligence deemed that the Leviathans themselves were part of the problem did they understand their error, but of course by then it was too late. The pawns of the Intelligence attacked and slaughtered most of the Leviathans, harvesting their genetic material to create a new machine in their image. This machine would come to be known as Harbinger, and with it the Intelligence would commence the cycle of harvesting the advanced organic races of the galaxy, which would continue for eons to come. With each race harvested, a new Reaper would be created that preserved the essence of the harvested race within its own structure.
A few of the Leviathans survived their creation's betrayal and went into hiding, and have remained hidden and isolated for millennia, observing the Reapers' harvest of the organic cultures that arise every 50,000 years. They have not felt inclined to intervene in any of the conflicts that have occurred, regarding the Reapers as simply fulfilling their purpose. Their only true focus for all that time was survival and over the many centuries they enthralled subsequent races to eliminate any evidence of their existence across the galaxy, for fear that the Reapers will come for them again. Using organic "artifacts" scattered on different planets, the Leviathans observed the Reapers' continuing cycles of genocide.
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, but only because they still consider themselves the galaxy's apex race and 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.