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 think of other races as mere 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.
Every new species they encountered was enthralled to serve them and provide "tribute", and in return, the thralls were cared for and protected. Over time, however, they observed that their thralls would frequently build synthetic constructs 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. The Leviathans acknowledge the Intelligence still serves its purpose as a tool, but they believe it has not fulfilled its purpose of preserving life.