WARNING: This page contains spoilers for the Mass Effect 3: Leviathan DLC.
The Leviathans are an ancient race of gigantic sapients from the Mass Effect series.
Leviathans are gigantic aquatic animals resembling terrestrial squids. They grow to hundreds of meters in length. They are comfortable at depths of thousands of meters, although they can apparently live at much shallower depths, as well.
Leviathans have the ability to indoctrinate or "enthrall" other intelligent life forms. The process takes time, but affected beings eventually become completely subservient to the Leviathans. The range of this power can be extended via artifacts created for the purpose. Such artifacts also serve as long range sensor devices for gathering intelligence.
While the Leviathans were once a space-travelling race that controlled most of the galaxy, what (if any) form of government they used is unknown. In the 22nd century, their numbers are apparently very small, requiring no formal government.
The Leviathians evolved to sapience more than 37 million years ago. It is unclear when they developed the ability to indoctrinate other life forms, but they presumably used it to control intelligent land animals on their planet, allowing them to create advanced technology that couldn't be made in the ocean.
Once they developed interstellar travel technology, the Leviathans spread to other worlds, indoctrinating any intelligent life they encountered to create a growing galactic empire.
Some species in their time developed synthetic life. The Leviathans observed that the synthetics inevitably rebelled against the organics that created them, resulting in the destruction of one or the other. As synthetics could not be indoctrinated, they created an artificial intelligence of their own to find a way to prevent synthetic life from wiping out organic life.
Accustomed as they were to controlling everything around them, the Leviathans were unprepared for their creation to turn against them. It determined that the way to preserve organic life was to destroy any species that could invent synthetic life before the inevitable war could occur. The AI, known in the 22nd century as the Catalyst, began "harvesting" species capable of creating synthetic life, starting with the Leviathans.
A few Leviathans survived the purge and went into hiding. It is not clear whether the Leviathans living in the 22nd century are the same ones that hid from the first harvest or their descendants.
In the 22nd century, enthrallment is the most significant threat that the Leviathans pose. This power is strong enough to affect Reaper creatures. Once Shepard convinces them to aid in the war against the Reapers, they provide enthrallment spheres which Citadel forces can place behind enemy lines to subvert the Reapers. The Leviathans also possess the ability to create pulses that disable nearby spacecraft, even Reaper capital ships. However, the Leviathans don't have sufficient numbers to openly oppose the Reapers; it is uncertain whether or not they even have the resources to attempt spacefaring at this point.