Cranium Rat

Most folk know the cranium rat as a pest of filthy villages: a rat whose swollen brain bulges from its skull and throbs with sickly light. One diviner who believed secrets escape sleepers each night claimed to have dreamed the swarm's own thoughts. It called itself the vishkar and boasted of watching the world through a thousand eyes, unseen by those it lives among. A month later the diviner was found dead, drained of blood through a hundred tiny bites.

Cranium rats are not aggressive. Like other vermin they prefer flight or ambush, and they ambush cleverly: packs of ten or more hide in refuse or wall cracks, swarm out when prey comes close, and scatter in every direction if the victim is not dead or crippled within a few rounds, making pursuit nearly hopeless.

Their real danger is the group mind. Each rat is in automatic telepathic contact with all others within 10 feet, and every five linked rats add 1 point of Intelligence. One to four rats have animal Intelligence (1), five are semi-intelligent (2), fifty think like an average person (10) and a hundred reach a frightening 20. No pack above about 20 has been found; perhaps bigger swarms ascend to some higher existence, or perhaps crowding brings a sudden die-off.

Against area spells, pool the pack's Hit Dice and ignore individual hit points: an 8-HD fireball kills eight of a 30-rat horde on a failed save, or four on a successful one. The pack saves as a creature with Hit Dice equal to its Intelligence, so those 30 rats (Intelligence 6) save as a 6-HD monster.

Losses are its weakness: as rats die or scatter, the pack's Intelligence and any powers drawn from the shared mind drop immediately. Yet the mind resists mental attack. A pack of Intelligence 5 or more is immune to sleep, and it quickly severs any rat that falls under outside control, so suggestion or charm monster affects only one rat, which still uses the pack's saving throw.

The diviner's notes describe a pulsing green thread linking the rats to a master that feeds on stolen secrets. He guessed they serve the great god-brain worshiped by mind reavers, acting as its eyes and ears. Cranium rats infest the great planar city and are said to be common in the Lower Planes. His last pages mention dreams of packs hunting for him.

They eat much like normal rats, if a bit more meat. Their purpose is kept carefully hidden to protect the god-brain's spies, and anyone who uncovers it faces a swift, terrible death.