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. Hit: 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.