In a nutshell, cranium rats earn their moniker for their enlarged brains, which visibly emerge from the tops of their heads. These mutant brains imbue the rats with powerful psionics - most notably, as they gather in numbers, they form a gestalt consciousness, which gets smarter and more psionically powerful with every additional cranium rat added to the swarm. Whilst they subsequently get dumber and weaker as members of the swarm are killed, a large swarm is a deadly foe.
When introduced in Advanced Dungeons & Dragons, cranium rats were the invention of the illithid god, Ilsensine, who used them as secret spies to spread throughout the planes and gather secrets for him, transmitting whatever they saw and heard back to the God-Brain in its lair. This plan was subtly opposed by The Us, a Sigil-based cranium rat swarm that had somehow broken free of its psychic link with Ilsensine and which sought to grow in power by subverting other swarms until it could kill and devour its former master.
Cranium rats reappeared in Dungeons & Dragons 5th Edition in the splatbook "Volo's Guide to Monsters". Here, they lost their divine servitor status, and were instead recast as a more down-to-earth creature; fleshcrafting-produced spies and minions of the illithids. Aside from that, though, they were faithfully translated.
|The inhabitants of the Planes of Planescape|
|Upper Planes:||Aasimon - Angels - Animal Lords - Archons - Asuras - Eladrin - Guardinals - Lillend|
|Middle Planes:||Formians - Githzerai - Inevitables - Marut - Modrons - Rilmani - Slaadi|
|Lower Planes:|| Alu-Fiends - Baatezu - Bladelings - Cambions - Demodands - Erinyes - Hags |
Hordlings - Imps - Kytons - Marilith - Obyrith - Succubi - Tanar'ri - Yugoloth
|Transitive Planes:||Astral Dreadnought - Githyanki|
|Inner Planes:||Azers - Elementals - Genies - Grues - Mephit - Salamanders - Sylphs|
|Sigil:||Dabus - Cranium Rats|
|High-ups:||Archangels - Archdevils - Archfey - Archomentals - Demon Princes|