The Leviathans are a primordial Lovecraftian-like race in the fictional universe of the Mass Effect video game.
The Leviathans are a primordial aquatic race that eons ago dominated the Milky Way galaxy. Whom saw themselves as the apex of evolution and that the purpose of other races were to serve them. Gigantic creatures whose form is similar to the squids or cuttlefish of Earth. With tentacle-like appendages extending from the sides and below the body, six eyes in the lower part, and bony, ridged plates covering the exterior.
Leviathans have the ability to influence lesser organisms up to the point of having complete mental and physical control. Communicating with others via telepathy and being able to turn them into thralls. It's theorized that they have cybernetic enhancements, as they are able to travel through space and communicate via interstellar distances using faster-than-light pulses. Leviathans have mastered organic technology, using a variety of "artifacts".
Billions of years ago, the Leviathans enthralled a land-dwelling species on their homeworld and used them to go to the stars and spread across the galaxy. Enthralling every species they encountered to serve the Leviathans and provide them with "tribute", in exchane for protection and having their welfare taken care of. As time went on they noticed that their thralls often built synthetic contructs who sooner or later rebelled, resulting in the extermination of many of their enthralled species.
The Leviathans decided to create the Catalyst, an artificial intelligence whose mandate was the preservation of life at all cost. Which built an army that was deployed throughout the galaxy to collect genetic data. It would eventually concluded that the Leviathans were part of the problem and betrayed the Leviathans. Slaughtering the vast majority of them and using their genetic material to create Harbinger, the first Reaper. Thereby beginning the Reaper cycle in the galaxy, as the surviving Leviathans were forced to go into hiding. From which they continued to observe the galaxy using organic "artifacts" and during 100s of millions of years, secretly erased evidence of their existence.