Swarm of Poisonous Snakes

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.

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.