This apparently disembodied brain moves about on four bestial, clawed feet, waving several curling tendrils. It is the size of a cat and moves with frightening alacrity. A glistening, transparent membrane or sheath covers its horrible body.

Intellect devourers are strange creatures of unknown origin, with bizarre feeding habits and a poorly-understood life cycle. They appear to begin life as sessile lumps of a pebbly, dark, fungus-like matter, much like large truffles; in this form, they live in clusters or colonies, seeming to prefer dark, moist environments, like caverns. At some point, they sprout a number of thin tendrils, which they use for locomotion and manipulation; they also gain limited telepathic or psychic ability, which seems to be somehow connected with the manner by which they gain nourishment.

The creature then goes through a growth period of some months, during which it increases in size, develops its protective membrane, and four of its tendrils slowly thicken and transform into clawed legs. Finally, after a short period of quiescence, the maturing intellect devourer has grown into its adult form, which looks very much like a humanoid brain with four clawed feet and four tentacles. At this point, the monster has a variety of mental powers, and requires the brains of living humanoids for its sustenance.