Multiattack

The swarm makes two Bites attacks.

Bites

Melee Weapon Attack: +7 to hit, reach 0 ft., one creature in the swarm's space. Hit: 21 (6d6) piercing damage, or 10 (3d6) piercing damage if the swarm has half of its hp or fewer. The target must make a DC 15 Constitution saving throw. On a failure, it takes 10 (3d6) poison damage and is poisoned for 1 minute. On a success, it takes half the damage and isn't poisoned. A poisoned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.