Cranium rats are minions of mind flayers, created “by bombarding normal rats with psionic energy” (and also, it seems from the illustration, delicately removing the top layers of their scalp and skull). Mind flayer colonies use them as forward observers; although the range of their telepathy is short (only 30 feet), the 120-foot telepathic range of a mind flayer extends their link to a more practical distance, and the 5-mile range of an elder brain increases their effectiveness by several orders of magnitude.
A lone cranium rat can’t do much. It’s extremely weak, with only 2 hp and 30 feet of darkvision. Its Bite attack is inconsequential. It can cause its brain to glow, emitting eerie dim light to a range of 5 feet, but that’s not very useful. If its range were 30 feet, it could combine this illumination with its darkvision to eliminate its Perception penalty within that radius. Since its passive Perception is only 10, that would be fairly useful. At only 5 feet, though, all a single cranium rat can do with this glow is give itself away.
However, a cranium rat that’s actively spying for a mind flayer colony might be ordered to use Illumination because a mind flayer or elder brain wanted to get a good, clear look at someone or something that the cranium rat had approached in the dark. In this scenario, the cranium rat is effectively doomed to die, providing a brief moment of extreme creepiness before it succumbs to an opportunity attack. If that attack should somehow miss, the cranium rat snuffs its light and Dashes away, hopefully to safety.
Cranium rats only pose a real threat when they get together in a swarm, which, like a hag coven, amplifies their abilities. In addition to their basic traits—and the traits common to swarms, which include various resistances and immunities and the ability to surround other creatures—cranium rats in a swarm manifest a highly intelligent hivelike consciousness, along with the ability to cast various spells psionically.
The most broadly useful of these spells is confusion, because it can affect multiple targets, and the closer the quarters, the more targets it can affect. Because confusion affects a spherical volume with a 10-foot radius, the swarm will never use it against just one target; the minimum is two. (If it wants to affect just one, it has command or dominate monster for that.) But since it’s only available once per day, the swarm applies it judiciously. As a swarm, cranium rats become intelligent enough to lead potential victims on, luring them into tight spaces and onto narrow paths flanked by treacherous drops. When as many targets as possible are enticed to group up within that 10-foot-radius sphere, that’s when they put the confusion whammy on them.
A swarm of cranium rats also has the additional ability to increase their cranial Illumination, shedding bright light up to a distance of 20 feet and dim light for 20 feet beyond that. In subterranean settings, this light (set to a radius of 15 feet rather than 20) is a great way to provoke the curiosity of incautious dungeon delvers: when those hapless mooks get within 30 feet, the swarm can use its bonus action to plunge the area back into darkness, then overrun its nearest victim and Bite. In contrast to the 1 piercing damage of a single cranium rat’s Bite, a swarm deals four dice of damage, which is far nastier.
Generally speaking, though, even a swarm of cranium rats doesn’t necessarily want a head-to-head fight with anyone. It’s happy to launch an unexpected shock attack, but it wants to finish that fight quickly, and if it doesn’t do enough damage in one round to vanquish its foe in one more, it pulls back, Disengaging—it’s smart enough to know how to do that—or, if allies are running to its target’s aid, takes advantage of their clustering to pop confusion. It’s generally pretty indiscriminate about target selection (except in one instance—see below), but if it’s targeted with a type of damage other than bludgeoning, piercing or slashing, it turns its fury against that opponent, using command to force it closer if it can’t close the distance on its own.
When in telepathic communication with a mind flayer, a swarm of cranium rats can provide much more useful intel than individual cranium rats can, using comprehend languages to translate other creatures’ conversations and detect thoughts to extend its master’s mind-reading range. It also has the smarts to scout independently, reporting back to a mind flayer or elder brain when it reestablishes a telepathic connection to check in. But one of the nastiest tricks up the swarm’s sleeve is dominate monster.
This stunt requires some delicacy to pull off. First, the target only has to be within 60 feet of the swarm—but the swarm has to be able to see them, and it has only 30 feet of darkvision. If the target or one of its companions is providing the light by which the swarm can see them, super! If, on the other hand, the target and its companions all have darkvision themselves and aren’t bothering to carry any light source, the cranium rats have to get closer, and they aren’t particularly stealthy.
Second, they have to correctly infer which of their possible targets is likely to fail a DC 13 Wisdom saving throw. Their Intelligence is good, but not extraordinarily good; their only means of evaluating their targets’ Wisdom is by following and observing them for a while, and even then, they’re making an educated guess at best. However, rats of normal size in a subterranean cavern are as common as copper pieces—certainly not something adventurers of modest renown and experience would concern themselves with. (Unless they happened to notice that the top of each rat’s head had been neatly sliced off. Can we disregard the illustration and agree that cranium rats look like normal rats, not like horribly unethical lab experiments?) A swarm of rats following a group of adventurers is weird, certainly, but if they eventually go away, there’s no need for violence to ensue, is there?
Anyway, if the swarm can stay close enough, long enough, to choose a promising victim and cast dominate monster on them, and if they fail the saving throw, that control can be maintained for up to an hour—and need not manifest outwardly in any way until the time is right. During that time, the swarm can have nice, friendly telepathic conversations with the target, inquiring into their abilities and their allies’ abilities, and consider how to make the best use of its new thrall. If they need a second opinion, a mind flayer or elder brain can provide that when it checks in. As long as the dominated target takes no damage, they don’t get another saving throw to escape being dominated. Also, the swarm no longer has to keep following the target: the telepathic link created by dominate monster is maintained as long as the target stays on the same plane of existence. (While the spell is active, the swarm has to maintain concentration and can’t use detect thoughts to read the mind of any other creature, nor cast confusion.)
Unfortunately for the swarm, if the target makes their saving throw, that’s it; the swarm doesn’t get another chance to cast dominate monster until the next day. But who’s to say a given mind flayer colony employs only one swarm of cranium rats?
Because of their darkvision, cranium rats prefer not to go out aboveground in daylight, and because the range of their darkvision is so short, they prefer confined spaces to open ones. Even in a swarm, cranium rats aren’t going to chase anyone through the market square at high noon.
A swarm of cranium rats in telepathic contact with an elder brain or mind flayer master will obediently fight to the death if ordered to. But what about one not under the influence of an elder brain or mind flayer, or under their influence but not in active contact with them? When reduced to 14 hp or fewer, the swarm scatters into six to eight individual cranium rats that all Dash off in different directions, unless the entire swarm can make it to a mousehole less than 60 feet away.
The question of what happens when a swarm of cranium rats moves beyond an elder brain’s range of influence is an interesting one, because for whatever it’s worth, cranium rats are still beasts; they’re not aberrations. Even a swarm of cranium rats isn’t an aberration—it’s a swarm of beasts. So does a swarm of cranium rats pursue the same goals as any other beast, only much, much more cleverly? Does it become unaligned, as most beasts are, or does it remain lawful evil? It may not be a mastermind, but it’s probably more than smart enough to, say, get itself elected to the Senate. Even if we assume its motivations remain fundamentally bestial, it’s still entertaining to contemplate how a swarm of cranium rats could use its respectable intellect not just to improve its access to food and defend its territory, but to guarantee its access to food and make its territory uncontestable—and cause no end of problems to anyone who tries to interfere with it. In a one-shot adventure for a party of level 3 or 4 characters, it could even be a worthy boss monster.
Next: slithering trackers.