Armor Class 12 (natural armor)
Hit Points 22 (4d8 + 4)
Speed fly 30 ft.
Damage Immunities poison, psychic
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned
Senses blindsight 60 ft. (blind beyond this radius), passive Perception 10
Languages —
Challenge 1/4
False Objects. If the swarm is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn't observed the swarm move or act, that creature must succeed on a DC 15 Wisdom (Perception) check to discern that the swarm is animate.
Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a 1-foot-tall, 8-inch-wide, 2-inch-thick object. The swarm can't regain hit points or gain temporary hit points.
Book Club. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 6 (2d4 + 1) bludgeoning damage, or 3 (1d4 + 1) bludgeoning damage if the swarm has half its hit points or fewer.
CM, p. 19