Armor Class 12
Hit Points 76 (17d8)
Speed 30 ft.
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned
Senses darkvision 30 ft., passive Perception 10
Languages telepathy 30 ft.
Challenge 5
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.
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.
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.
Mordenkainen Presents: Monsters of the Multiverse, p. 83