A pest across the planes, the cranium rat is a vermin with a secret. A strange sight, these look like nothing more than common rats with the tops of their skulls lopped off, letting their brains pulse out through them. However, cranium rats are in fact the eyes and ears of the god-brain Ilsensine, spread throughout the planes to gain ever more information for the power. Though a few hives do fall out of its control, most notably the Us of Sigil, in general these vermin serve as one of many information-gathering sources for the great deity of the illithid. Few are aware of these origins, however, and even fewer survive to inform others once they discover the truth.
Cranium rats are almost always found in groups, for it's in groups that their greatest potential is found. The more cranium rats in close proximity to one another, the greater the intelligence of the lot, creating a powerful hive mind that rapidly gains in mental abilities as they expand their minds. Though a single cranium rat is no more brighter than a normal rat, a large swarm quickly grows in intelligence to rival the most brilliant of humanoids.
When facing others, a hive of cranium rats prefers to keep hidden in the shadows, the crannies of walls, or the refuse along the edges of alleyways, setting up an ambush if it desires to take on a target for whatever reason. Though the central hive will usually remain secluded somewhere safe and most often underground, they will often send out small scouting parties to do Ilsensine's will and gather information; usually groups of at least 10, though more are often dispatched if greater capabilities are necessary amongst the group.
Cranium rats can be found across the Outer Planes, though they are most often found within the Lower Planes, quickly being removed as the pests and dangers they are in the Upper. They are especially tenacious within Sigil as well, with the Department of Vermin Control often hiring groups to exterminate new hives found in various sections of Undersigil.
- Planescape Monstrous Supplement, pp.8-9