A Great Brain looks like a monstrous, immobile brain ringed by many dexterous but weak tentacles, which it uses both as manipulators and to drag itself around. The exact relationship of the great brain to thulids is unknown. A great brain does not speak, but if it has need for it, can telepathically communicate with any creature of greater than animal intelligence.

Although slow and physically weak, a great brain possesses great magical power, and it shares the thulid’s ability to use a mental blast with the same effects.