Bat, Swarm of Bats

Cacophonous Swarm. A creature in the swarm's space that casts a spell with a verbal component must succeed on a DC 12 CON save or the spell fails to cast, expending the action required to cast it but not the spell slot or daily use.

Echolocation. The swarm can't use its keensense while deafened.

Heightened Hearing. The swarm's Perception is 16 when perceiving by hearing.

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 bat. The swarm can't regain HP or gain temporary HP.

Swarm Resilience. The swarm is resistant to bludgeoning, piercing, and slashing damage, and it is immune to the charmed, frightened, grappled, paralyzed, petrified, prone, restrained, and stunned conditions.

Bites. _Melee Weapon Attack:_ +4 to hit, reach 0 ft., one creature in the swarm's space. _Hit:_ 7 (2d6) piercing damage, or 3 (1d6) piercing damage if the swarm has half of its HP or fewer.