Cranium Rat And A Swarm of Cranium Rats

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.

Basic Information

Anatomy

Beast, Cranium Rat:

Additional Information

Perception and Sensory Capabilities

• Darkvision 30ft, and 
• Passive Perception 10

Origin/Ancestry

Beast (Rat)

Cranium Rat CR: 0 (10XP)

Tiny beast, lawful evil

Armor Class: 12

Hit Points: 2 (1D4)

Speed: 30ft

STR

2 -4

DEX

14 +2

CON

10 +0

INT

4 -3

WIS

11 +0

CHA

8 -1

Senses: Darkvision 30ft, and Passive Perception 10.

Languages: Telepathy 30ft.

Illumination

As a Bonus Action, the cranium rat can shed dim light from its brain in a 5-Foot radius or extinguish the light.

Telepathic Shroud

The cranium rat is Immune to any Effect that would sense its emotions or read its thoughts, as well as to all Divination Spells.

Actions

Bite

Melee Weapon Attack: +4 To Hit, Reach 5ft, One Target. Hit: 1 Piercing Damage.

A Swarm of Cranium Rats CR: 5 (1,800XP)

Tiny swarm of beasts, lawful evil

Armor Class: 12

Hit Points: 36 (8D8)

Speed: 30ft

STR

9 -1

DEX

14 +2

CON

10 +0

INT

15 +2

WIS

11 +0

CHA

14 +2

Damage Resistances: Bludgeoning,Piercing and Slashing

Condition Immunities: Charmed,Frightened,Grappled,Paralyzed,Petrified,Prone,Restrained and Stunned

Senses: Darkvision 30ft, and Passive Perception 10.

Languages: Telepathy 30ft

Innate Spellcasting (Psionics)

The Swarm’s innate Spellcasting Ability is Intelligence (Spell Save DC 13). As long as it has more than half of its Hit Points remaining, the swarm can Innately cast the following spells, requiring No Components.

At will:

• Command
• Comprehend Languages
• Detect Thoughts

1/day:

• Confusion
• Dominate Monster

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 5-20-Foot Radius (And Dim Light for an additional number of feet equal to the Chosen Radius), or Extinguish the Light.

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.

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.

Actions

Bite

Melee Weapon Attack: +5 To Hit, Reach 0ft, 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. 

Suggested Environments

The Underdark.