Insect, Swarm of Insects

Crawling Swarm. A creature in the swarm's space is covered in crawling and buzzing insects. If the creature casts a spell with a somatic component, it must succeed on a DC 11 CON save or the spell fails to cast, expending the action required to cast it but not the spell slot or daily use.

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 insect. 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:_ +3 to hit, reach 0 ft., one target in the swarm's space. _Hit:_ 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half of its HP or fewer.