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 hit points or gain temporary hit points.
Bites. Melee Weapon Attack: +5 to hit, reach 0 ft., one target in the swarm's space. Hit: (4d4) piercing damage, or (2d4) piercing damage if the swarm has half of its hit points or fewer.