Intellect devourer

Overview

Intellect devourers were psionic aberrations that took the form of walking brains. They were servants of the mind flayers and were one of the most dreaded threats of the Underdark, with their power to consume intelligence and possess host bodies. They looked like little more than disembodied large humanoid brains that could walk on a set of four bestial double-jointed bony legs with feet bearing three stubby talons. Though their size was variable thanks to their psionic compression power, they ranged in size from 6 inches (15 centimeters) long, to about as big as dogs. Despite their appearance, they could move and attack with shocking speed. They made warbling sounds while moving.  

Combat

  They were capable of magically consuming a humanoid creature's mind and memories from a distance of up to 10 feet (3 meters), provided that creature had a brain and succumbed to its psychic assault. If so, the victim's mentality was deteriorated. In the worst case, the victim's intellectual capacity was completely erased, leaving them dazed or stunned and ultimately incapacitated. Psionic intellect devourers instead absorbed mental energy as with psychic drain or consumed the victim's confidence, as with the ego whip power. If the intellect devourer approached a vulnerable victim (whether incapacitated or already devoured) within 5 feet (1.5 meters), it could then battle the brain's remaining defenses; a protection from evil spell prevented this and repelled them. If successful, it magically or psionically consumed the physical brain, then teleported inside the empty skull to replace the brain, whereupon it could use the body as a host, as per dominate person. Naturally, this killed the victim. A fresh corpse could also be used as a host, but not undead and other kinds of creatures not especially hampered by the loss of a brain. Any difference in size was taken care of by their compression power.