Ecology: Intellect devourers live deep underground, or in dark wilderness lairs. They abhor bright light and if outside are only encountered on dark nights. Underground they are active at all times. They live off of psychic energy, and will feed on almost any thinking being, although the more intelligent, the better. Little else is known about these creatures, other than that they are very inimical to almost all forms of life.
Intellect devourer's are very intelligent, and possess a great cunning. As they desire to feed of of psychic energy, they tend to be crafty while hunting, preferring to lie in wait and ambush the unsuspecting, as opposed to fighting openly. Their near invulnerability to many forms of attack does make then somewhat arrogant, and an intellect devourer may attack a large group of small creatures, trying to slay as many as possible in order to feed off of the energy released by the creatures deaths.
An intellect devourer tends to stalk a single victim, seeking to trap the target alone. Their initial assault will then be a psychic crush, which will hopefully stun their target. This is followed by Ego Whip, which will then prepare the victim for mental possession by the devourer. The devourer will then seek out the victim's allies and friends and try to repeat the process.
This write up was tough, as I had to try and make sense of AD&D's rules for psionics. The problem is, the devourer can kill it's target with it's psionic attacks and then inhabit the body that's left. One could do this in Hero System, but it would require EGO Attack to have the "Does BODY" Advantage. The Mind Transference power comes from The Ultimate Mentalist, and allows the intellect devourer to swap minds with it's target. In effect, the devourer inhabits its victim's body, while mind of its victim lives in the devourer's comatose form. For more info, see the The Ultimate Mentalist.
An intellect devourer appears to have a huge brain (usually said to appear like a human brain) or brain-like body, slung between four powerful and clawed legs. The body is whitish, while the legs are usually a light tan. The devourer has no visible eyes, ears, nose or mouth. It is unknown what it eats, but it is believed to subsist of the mental energy of others.