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 snake. The swarm can't regain Hit Points or gain Temporary Hit Points.
Bites. Melee Attack Roll: +6, reach 5 ft. Hit: 8 (1d8 + 4) Piercing damage—or 6 (1d4 + 4) Piercing damage if the swarm is Bloodied—plus 10 (3d6) Poison damage.