Leviathans

Also known as Aboleth, leviathans are ancient, sea-dwelling beings that many believe to be the very first to achieve true sentience. Like the other ancient species that comprise the Primordial Titans, the leviathans are now but a mere echo of what they once were.
Even the name "leviathan" is scarcely used, as the serpent-like aboleth have lost their immense stature over time; small compared to even modern giants and dragons of the present age. What sets them apart is their collective memory. Aboleth have developed potent psionic abilities, and each member of their species shares a sort of collective consciousness, with each individual able to access the memories and experiences of every previous Aboleth, all the way back to their first awakening in the depths of the primordial seas. 
Despite their decline, Aboleth continue to pose an existential threat to all who encounter them. Their psionics and unique physiology allow them to beguile and mutate other sapient races into their thralls. Ancient relics and fossils indicate that leviathans ruled over vast empires of proto-humanoids through these abilities back when only a fraction of the world was dry land. 
It is unknown how many Aboleth still exist in the modern day, but many scholars aware of their abilities live in fear of the day they finally have the numbers and determination to retake their long-lost domain and press all mortal kingdoms into servitude.