Caverns of Thought

The Caverns of Thought were the divine realm of Ilsensine, the illithid god of magic.

Geography
According to the Great Wheel cosmology, the Caverns of Thought were located deep underground beneath the Outlands and were connected to the realm of the beholder deity Gzemnid. The World Tree cosmology established the realm as part of the fiendish plane of the Deep Caverns.

In both cosmologies, however, the Caverns of Thought were described as a hostile place wrought with inaudible yet clear and powerful thoughts that slowly drove its visitors mad until they were reduced to thoughtless zombies. This effect was a consequence of Ilsensine's power prying the minds of the realm's visitors in search of their secrets.

The realm was a labyrinth of tunnels that were lit by the faint glow of fungus. It was avoided even by fiends. The dwarf inhabitants of the nearby realm of Dwarven Mountain also avoided digging their tunnels too close to the Caverns.

The only creatures that dared visit the realm were motivated by the search for knowledge that Ilsensine possessed. However, the price he demanded was high, often leading to the visitors' losing part of their own minds.