SQDSMP

Intellect devourers are small, brain-like creatures with four clawed legs that live in the Underdark with mind flayers. They are chaotic and evil monsters that feed on the psychic energy of sentient creatures. Mind flayers use intellect devourers as pets or hunters. They can also be used as spies because they can take over humanoid bodies and move around undeterred.

Mind flayers create intellect devourers by performing a ritual on a thrall’s brain to make it sprout legs. This ritual also seems to infuse the brain with psionic abilities not unlike that of a mind flayer.

Intellect devourers have a special ability that allows them to target a creature's intelligence and eat its brain. If successful, they replace the creature's brain with their own, gaining control of the body. This is done magically, so it's not physically possible to tell if a creature is being controlled by an intellect devourer.

Rarely, an intellect devourer that lived a long time and had grown massive on consumed intelligence underwent a second metamorphosis, becoming what was known as an intellect glutton. These creatures were much more powerful than their previous form but could no longer take on a creature as a host due to their size.