Swarm of Quippers

The swarm has advantage on melee attack rolls against any creature that doesn't have all its hit points.

The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny quipper. The swarm can't regain hit points or gain temporary hit points.

The swarm can breathe only underwater.

Melee Weapon Attack +5 to hit, reach 0 ft., one creature in the swarm's space. 14 (4d6) Piercing damage, or 7 (2d6) Piercing damage if the swarm has half of its hit points or fewer.