An ancient alien race seeking to conquer the multiverse. Masters of the Astral Plane and Limbo and masters of Psi powers - yet also no strangers to magic or technology either. Almost universally considered a priority common enemy by even deeply-opposed forces.
Brain eaters in both the literal and power-effect sense: mentally enslave their subjects - have minions and rebels in the enslaved races of the Gith (Githzerai and Unknown).
https://forgottenrealms.fandom.com/wiki/Mind_flayer