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 species 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.
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 ventral 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. Members of the race have been witnessed at depths of over 3200 meters.
The Leviathans possess the natural ability to influence the behaviors of lesser-minded organisms to the point of 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 . They might even have modified themselves with biotic amplifiers. In addition, their faster-than-lightcommunication pulses require precise control with technology completely unknown to modern science.