Intellect devourers, also known as brain-dogs or rochnon, were psionic aberrations that took the form of walking brains. They appeared as large, disembodied 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 to about as big as Dogs.
Their brain matter was protected under a crusty coating or a glistening, clear membrane, that resisted physical harm. They often emmited strange warbling sounds, and could detect sentience around them up to a long range. This detection was coupled with a telepathic ability to communicate simple ideas, but they could not truly speak languages unless in possession of a host.
Intellect devourers were typically servants of the Illithid, and with power to consume intelligence and possess host bodies, were one of the most feared creatures in the Underdark. Despite their appearance, they could move and attack with shocking speed. They were capable of magically consuming a humanoid creature's mind and memories from a distance of up to 10 feet, 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.
If the intellect devourer approached a vulnerable victim, it could then battle the brain's remaining defenses. 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. Though this process killed the victim, they required a living host, so they could not take over an Undead body. Once in control of its host, an intellect devourer had full control of the body, while the devourer retained its own mental abilities. They also gained knowledge of their host, though could not identify specific memories.