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

Bites.

Melee Weapon Attack: +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, plus 22 (5d8) psychic damage.

Spellcasting (Psionics).

As long as it has more than half of its hit points remaining, the swarm casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 13):

At will: command, comprehend languages, detect thoughts
1/day each: confusion, dominate monster

Bonus Actions

Illumination.

The swarm sheds dim light from its brains in a 5-foot radius, increases 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 extinguishes the light.

Environment: Underdark, Urban

Swarm of Cranium Rats