Illithid

More commonly known as "Mind Flayers".

Nightmarish, alien invaders from another time and place, illithid typically live in societies of their own kind, hidden away from the light of the sun. They feed on the brain matter of living creatures, preferably the grey cells of other sapient beings, to sustain themselves and fuel their strange and unnatural powers. Notorious as the "mind flayers" on dozens of worlds, the illithid are possessed of a desire to dominate and master all things around themselves, and they famously use their advanced mental powers to force other beings into thralldom.

Still, members of illithid-kind can and do take up the adventuring life, from time to time, though almost always under rare and extraordinary circumstances. Leaving the collective networked intelligence of their own kind diminishes any illithids that brave the outside world, severely reducing the potency of most of their natural psionic abilities. Most illithids encountered outside their citadels and outposts are either spies, renegades unwelcome in their communities, or have been banished for crimes even their alien moralities find unspeakable.

Find out more here: https://www.dandwiki.com/wiki/Illithid_(5e_Race)

Civilization and Culture

Interspecies Relations and Assumptions

The Rock of Bral and Prince Andru

The illithids are frighteningly polite to the people of the Rock. In general, Andru enjoys about the best human—mind flayer relations to be found anywhere. Most Bralians agree that this is because even the mind flayers need a neutral ground on which to conduct dialogues with other races, and Bral is one of the few places that will allow them to peacefully conduct their business.

Other Sources:
DandD Wiki