Ancient enemy of the Aasimar, likely corruptor of them into the Angels given the squid faces the angels now sport. Their empire was also known as the Beast of a Thousand Eyes, metaphorically. Famously psychic corruptors who enslaved all they encountered. The Fade Beasts are their tools.
One was found imprisoned half-dead beneath the Galoring Cathedral, captured by Tyr Isendel in one of the aasimar ruins with others that were supposed to be held in chambers (but the magic sustaining them failed and they were dead).
Note
Pris note: I forget what happened to it - did it teleport away? Was it killed?
Ancient references exist that speak of the illithid, describing them (and also calling them mindflayers), and mentioning a floating island called Xerastul’n.
Rhun Enkerig has told the Dreamers something of his life growing up as a slave to the illithid, overthrowing them and their Elder Brain, and about ceremorphosis.