Psychic Messenger.

The swarm can use its sending spell to target someone familiar to a creature in telepathic contact with the swarm.

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 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).

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 *, detect thoughts *, sending *
1/day each: confusion *, dominate monster *

*To cast this spell, the swarm must have more than half its hit points remaining.

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.

Cranium Rat Squeaker Swarm