Mind Flayer

These aberrations come in many variations. Most have similar features - tentacles, slimy body, and psionic abilities. Many are humanoid in appearance but with an octopus-like, ridged head with many tentacles surrounding a lamprey-like mouth.  They are warm-blooded amphibians, whose blood has a silvery-white color. Others sport an insurmountable number of tentacles growing out of many parts of their body or sprouts wings. Most have hands that have long, reddish fingers and lacking the index finger, and their feet are two-toed and webbed. Others have a sizable number of tentacles instead of hands and legs. Mind flayer eyes are extremely sensitive to bright light, and they consider it painful, a characteristic at odds with many other sources about the Undertethys or their origins. Their vision is also more sensitive to the recognition of geometric patterns than that of other humanoids. Mind flayers that are healthy from brain-rich diets excreted a kind of slimy mucous substance that coated their mauve skins.
Nobody knows where the Mind Flayers come from. Named as Star Children, Star Spawn (not to be mistaken with Star Spawn), Children of the Hidden Lord, and many other titles trying to understand their origins. One story tells a tale about the First Gods before the First Age, The Titan War, where the city of Penumbra that surrounded Sol. The gods conquered the system, destroying the city, leaving only the Ring of Dust. Those that escaped were able to get to Tethys. The second major story is about them being created by the Hidden Lord during the Second Age, Godswar. They rebelled against their lord, creating their own gods, becoming a major force during the Godswar. Other stories are about them being a starfaring race that comes to enslave every living being. No matter their origins, the Mind Flayers were a great force for evil in the Second Age, Godswar. Many empires were created by them, most staying near the shores and mostly fighting Elves and Dwarves. Their gods were killed during the war, most notable was Lugrighrost, whose fall destroyed a major Mind Flayer empire in the Old World. When the Third Age, Kingdoms dawned, their nations were destroyed and the race was hunted. Most would survive by going deep underground, into the Undertethys and creating their own empires there. It is said that they plot to return and enslave all mortals for defying their rule.

On Tethys

Almost all Mind Flayers can be found in Undertethys, where they plot and scheme. Their aberrant minds feed on others, especially intelligent humanoids. Brain matter is prime food for them and they would conduct many hunting parties or buy slaves to feast on. They worked with the Duergar and Drow on some occasions but all the time it's more calculated risk and reward than something that could flourish into an alliance. 

First World

In the First World, the Mind Flayers were a sinister, hidden race of psionics that were unknown to the majority of the races. They were created by a powerful deity deep underground.