Individually, cranium rats are little more than your ordinary garbage rats with the slight addition of pulsing, exposed brains on their skulls. Get them in a group, though, and they become much more dangerous - their proximity to other cranium rats creates a hive mind that grows more powerful with the addition of every rat. Their intelligence can rapidly increase to godlike level, and they gain spells and other fearsome mental abilities - and it's only by slaying numerous individual rats that you can weaken the hive mind. Best to avoid them, if at all possible.
Cranium rats are the creations of the Illithid god Ilsensine.
Sources:
Planescape Campaign Setting
Fiend Folio (D&D 3.5)
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