Amphibious. The swarm can breathe air and water.
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 eel. The swarm can’t regain hit points or gain temporary hit points.
Bites. Melee Weapon Attack: +3 to hit, reach 0 ft., one creature in the swarm’s space. Hit: 14 (4d6) piercing damage, or 7 (2d6) 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) psychic damage on a failed save, or half as much damage on a successful one.