Cranium rat

Cranium rats were psionically enhanced rats, transformed by mind flayers by bombardment with psionic energy.

Behavior
When isolated, cranium rats behaved like ordinary rats, using their telepathy to communicate basic feelings. When forming a swarm, however, the collective intelligence of the rats was combined into a single collective entity, which could also access enhanced psionic abilities from the mind flayer that created them. In such cases, a swarm of cranium rats could use psionic powers similar to spells.

Ecology
Cranium rats were used as spies for mind flayer colonies. They infiltrated surface communities and read the inhabitants' thoughts, later transmitting them to the elder brain.