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 Weapon Attack: +6 to hit, reach 0 ft., one target in the swarm's space. Hit: 7 2d6 piercing damage, or 3 1d6 piercing damage if the swarm has half of its hit points or fewer. The target must make a DC 10 Constitution saving throw, taking 14 4d6 poison damage on a failed save, or half as much damage on a successful one.