The Leviathans are an ancient aquatic race in the Mass Effect Universe that dominated the galaxy in the time before the Reapers. 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.
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, and observe the galaxy.