Swarm of Rot Grubs

The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny maggot. The swarm can't regain hit points or gain temporary hit points.

Melee Weapon Attack: +0 to hit, reach 0 ft., one creature in the swarm's space. Hit: 7 (2d6) piercing damage, and the target must succeed on a DC 10 Constitution saving throw or be poisoned. At the end of each of the poisoned target's turns, the target takes 3 (1d6) poison damage. Whenever the poisoned target takes fire damage, the target can repeat the saving throw, ending the effect on itself on a success. If the poisoned target ends its turn with 0 hit points, it dies.