Tales of Tolgard
Cranium rats are verminous creatures not uncommon in Sigil and pestilent cities of the Lower Planes. The magical beasts are distinguishable from their normal cousins by the pulsating brains that protrude from the tops of their skulls. They are many creatures and one all at once, for they possess a type of group mind.
Cranium rats exist as the eyes and ears of Ilsensine, the great god-brain of the illithids. The extent and purpose of their powers are held closely secret, lest Ilsensine’s instruments be discovered. Those who discover the true purpose of the cranium rats are under sentence of swift and terrible death.
The cranium rat is not an aggressive creature, avoiding open attacks in favor of flight or ambushes. Cranium rats usually move in swarms of ten or more.