Swarm of Sorrowfish

The swarm can occupy another creature's space and vice versa, and the swarm can move through an opening as narrow as 1 foot wide. The swarm can't regain hit points or gain temporary hit points.

Each time the swarm takes damage, any creature within 5 feet of it must make a DC 13 Wisdom saving throw. On a failed saving throw, the creature suffers the following effects until the end of its next turn: it has disadvantage on its attack rolls, it can't take reactions, and its speed is halved.

The swarm can breathe only underwater.

Melee Weapon Attack +8 to hit, reach 0 ft., one target in the swarm's space. 21 (6d6) Piercing damage, or 10 (3d6) Piercing damage if the swarm has half its hit points or fewer.

Each creature in the swarm's space must make a DC 13 Wisdom saving throw. On a failed saving throw, a creature takes 24 (7d6) Psychic damage and is Stunned until the end of its next turn. The effect ends on a creature if the swarm moves out of the creature's space or if another creature within 5 feet of the swarm uses an action to make a DC 14 Strength check, pulling the affected creature out of the swarm on a successful check. On a successful saving throw, a creature takes half as much damage and suffers no other effects.