Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through an opening large enough for a Medium creature. The swarm can't regain Hit Points or gain Temporary Hit Points.
Bites. Melee Attack Roll: +4, reach 5 ft. Hit: 9 (2d6 + 2) Necrotic damage, or 7 (2d4 + 2) Necrotic damage if the swarm is Bloodied.
