Homebrew Cranium Rat Swarm Species Details

Mind flayers create cranium rats by bombarding normal rats with psionic energy.

Evil Collectives. Cranium rats are no smarter than ordinary rats and behave as such. However, if enough cranium rats come together to form a swarm, they merge their minds into a single intelligence with the accumulated memories of all the swarm's constituents. The rats become smarter as a result, and they retain their heightened intelligence for as long as the swarm persists. The swarm also awakens latent psionic abilities implanted within each cranium rat by its mind flayer creators, bestowing upon the swarm psionic powers similar to spells. A rat separated from the swarm becomes an ordinary cranium rat with an Intelligence of 15. It loses 1 point of Intelligence each day that it remains separated from the swarm. Its Intelligence can't drop below 4 and becomes 15 again if it rejoins the swarm or another one.

Telepathic Vermin. A single, low-intelligence cranium rat uses its natural telepathy to communicate hunger, fear, and other base emotions. A swarm of cranium rats communicating telepathically "speaks" as one creature, often referring to itself using the collective pronouns "we" and "us."

Spies for an Elder Brain. Mind flayer colonies use cranium rats as spies. The rats invade surface communities and act as eyes and ears for the elder brain, transmitting their thoughts when they swarm and are within range of the elder brain's telepathy. Cranium rats occasionally spread beyond the elder brain's range of influence. Whatever these rats do is of no concern to the elder brain, and the illithids can always make more if they so desire.

Cranium Rat Swarm Traits

Racial traits are darkvision, illumination, swarm, telepathic shroud, bite, natural inability

Darkvision

You can see in dim light within 30 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Innate spellcasting (psionics)

The swarm's innate spellcasting ability is Intelligence. As long as it has more than half of its hit points and is of the appropriate level, it can cast the following spells, requiring no components:

Illumination

As a bonus action, the swarm can shed dim light from its brains in a 5-foot radius, increase the illumination to bright light in a 5 to 20-foot radius (and dim light for an additional number of feet equal to the chosen radius), or extinguish the light.

Illithid Origin

Having been created by Mind Flayers, you can speak Undercommon and Deep Speech.

Swarm

The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can't regain hit points or gain temporary hit points. In addition, the swarm is immune to the grappled, prone, restrained, and stunned conditions; has advantage on saving throws against being charmed, frightened, paralyzed, or petrified; and is resistant to bludgeoning, piercing, and slashing damage (even if the weapon is magical).

Bite

+5 to hit, reach 0 ft., one target in the swarm's space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.

Telepathic Shroud

The swarm is immune to any effect that would sense its emotions or read its thoughts. as well as to all divination spells.

Many Tiny Bodies

You are a swarm of tiny rats. You cannot wield weapons or wear armor.