Cranium rat

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.

Cranium rat: AC 6; MV 15; HD 1; THAC0 19; #AT 1; Dmg 1d4; SA See below; SD See below; MR Variable; SZ T (6" long); ML Unsteady (7); Int Animal to supragenius (1-20); AL NE; XP 65.

Cranium rats are the eyes and ears of the god-brain Ilsensine, spread throughout the planes to gain ever more information for that being. Though a few hives do fall out of her 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.

Appearance

Cranium rats are almost always found in groups, for it's in groups that their greatest potential is found. The more cranium rats are in close proximity (at least 10 feet), the greater the intelligence of the lot, creating a powerful hive mind. Though a single cranium rat is no more brighter than a traditional such pest (Int 1), every 5 rats within range of one another generate another point of intelligence for the lot, to a maximum of 20.

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 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.

Abilities

Intelligence	Ability
1-6	Standard
7	1 spell level of wizard spells
8	2 spell levels of wizard spells
9	Mind blast, 1/3 rounds
10	3 spell levels of wizard spells
11	4 spell levels of wizard spells
12	Mind blast, 1/2 rounds
13	5 spell levels of wizard spells
14	6 spell levels of wizard spells
15	Mind blast, 1/round
16	Immune to gases
17	Immune to cold
18	10% magic resistance
19	40% magic resistance
20	70% magic resistance

The level of ability a cranium rat hive possesses is directly tied to its Intelligence, and thus its size. First off, when calculating area damage against a hive of cranium rats, individual hit dice are ignored, and instead the hive is treated as a massive pool. For example, against an 8-HD fireball, if the hive fails its save, 8 rats will be slain; if it passes, only 4 will. A hive saves as a creature of as many HD as it has Intelligence at the time; a horde of 30 rats saves as a 6 HD creature, for example.

Coming from illithid origins, cranium rats of course also gain mental abilities as they gain in intelligence. At higher intelligences, cranium rats gain the ability to cast a certain number of levels worth of wizard spells per day, mind blast as an illithid, and at the highest levels, gain immunities to certain attack forms and resistance to magic through sheer force of will. Thus, dispersing a hive is by far the most effective means of dealing with one; even cutting a hive into two smaller groups halves the capabilities of both sides at worse. Mental methods of such are fairly ineffective, however, as any mental control is made only over an individual within the hive, to which telepathic contact is quickly cut when such a spell takes effect. Thus, spells such as suggestion or charm monster, though technically effective, are poor ways of dealing with a cranium rat hive.

References

• Planescape Monstrous Supplement, pp. 8-9