Balhannoths had an ovoid body, six tentacles, and a large mouth of jagged teeth.
Balhannoth were blind creatures, but capable of sensing magical energies from up to 120 feet (37 meters) away, whether the energies were from spells or magical items - though they could not distinguish between the two. Though other sources claimed that they could sense prey from up to 500 feet (150 meters) away.
Balhannoths were most notable for being capable of warping reality around them in a square area of up to 500 feet (150 meters) on a side for around 10 minutes. However, this was limited in its scope as they were incapable of creating objects with moving parts or magical properties. Furthermore, balhannoths could only affect nonliving objects and close inspection of objects within a warped area would reveal them to be facsimiles.
Balhannoths were capable of telepathically reading other creatures minds. They would use this ability to reach out towards any intelligent creatures that got within 1 mile of their lairs and learning whatever it was they desired most. Those that did would begin feeling as though they were close to that desire and this feeling would grow stronger as they got closer to its lair. They would then use their reality warping abilities to make their lair resemble what the creature desired.
Once it lured a creature within its lair a balhannoth would grapple and teleport them anywhere within 60 feet (18 meters) to begin their attack in earnest. Though if a balhannoth was not yet ready to go on the offensive, they were capable of turning invisible, altering their skin so that it blended in with their environment.
Rather than hunting for prey the balhannoth were known to be ambush predators. Balhannoth typically attacked their prey by biting with their large jagged mouths or grappling and constricting with their tentacles. While being grappled by it, a balhannoth's prey were rendered incapable of casting spells and the properties of any magical items on their person were suppressed.
They were a sexless species and mated with other members of their species by intertwining tentacles. It took over a year following sex for a balhannoth to lay their egg, after which they had an incubation period of one month. While incubating, a balhannoth egg needed to be kept near some form of magical item or magic area of effect. Once a balhannoth was hatched they reached maturity within three years.
Balhannoths fed on both flesh and magical auras, such as from magical items. The stronger a magical aura that a creature they feasted on possessed, the longer they would be sustained by it. They typically fed upon one magical item at a time, absorbing their energies, while storing others in their lairs within either ceiling cracks or beneath large boulders. Eventually they grew tired of the taste a magical item gave off and would feel it necessary to seek out a new one.
Drow and many other creatures within the Underdark were known to venture into Shadowfell border crossings in search of balhannoths to enslave as they were considered excellent guards. Other Underdark races known for capturing balhannoths for use guards included Mindflayers, aboleths, and even grimlocks. Though some, such as kuo-toas, instead raised balhannoths from birth.
Though they could be trained to respond to spoken commands, balhannoths often responded best to telepathic communication.
- Mike Mearls, Jeremy Crawford (May 29, 2018). Mordenkainen's Tome of Foes. Edited by Kim Mohan, Michele Carter. (Wizards of the Coast), p. 119. ISBN 978-0786966240.
- Mike Mearls, Stephen Schubert, James Wyatt (June 2008). Monster Manual 4th edition. (Wizards of the Coast), p. 24. ISBN 978-0-7869-4852-9.
- Gwendolyn F.M. Kestrel (July 2006). Monster Manual IV. (Wizards of the Coast), p. 15. ISBN 0-7869-3920-6.
- Gwendolyn F.M. Kestrel (July 2006). Monster Manual IV. (Wizards of the Coast), p. 16. ISBN 0-7869-3920-6.
- Mike Mearls, Jeremy Crawford (May 29, 2018). Mordenkainen's Tome of Foes. Edited by Kim Mohan, Michele Carter. (Wizards of the Coast), p. 118. ISBN 978-0786966240.