A Mind is a Terrible Thing to Waste

"Mind flayers, also known as illithids, are horrific, alien humanoids that lurk deep within the Underdark. Masters of psionic energy, they use their mental powers to dominate other creatures. The fortunate among their victims are slain, their brains devoured. The unlucky ones have their psyches warped, leaving them as mindless slaves with little hope of being rescued."

--Volo's Guide to Monsters