Grimmlet Swarm

Enervating Maelstrom. When the grimmlet swarm dies, it explodes in a plume of ennui. Each creature within 20 feet of the grimmlet swarm must make a DC 17 Dexterity saving throw. On a failure, a creature takes 21 (6d6) psychic damage and suffers one level of exhaustion. On a success, a creature takes half the damage and doesn't suffer exhaustion. Grimmlets damaged by this trait don't Reproduce.

Maze of Edges. A creature that attempts to move out of or through the grimmlet swarm must succeed on a DC 15 Dexterity saving throw or take 9 (2d8) slashing damage.

Reproduce. When a grimmlet swarm takes damage from a spell and isn't reduced to 0 hp, a number of new grimmlets equal to the spell's level appear in unoccupied spaces within 10 feet of the grimmlet swarm. If the spell is a cantrip, only one grimmlet is created. New grimmlets aren't subsumed into the swarm. Sixteen or more new grimmlets within 30 feet of each other can use their reactions to come together and form a new grimmlet swarm in a space within 5 feet of one grimmlet.

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

Innate Spellcasting (Psionics). The grimmlet swarm's innate spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). It can innately cast the following spells, requiring no components: At will: maddening whispers, crushing curse, minor illusion 3/day each: hypnotic pattern, void strike, major image 1/day each: hallucinatory terrain

Multiattack. The grimmlet swarm makes two attacks with its crystal edges.

Crystal Edges. Melee Weapon Attack: +8 to hit, reach 0 ft., one creature in the swarm's space. Hit: 18 (4d8) slashing damage, or 9 (2d8) slashing damage if the swarm has half of its hp or fewer. The target must make a DC 17 Intelligence saving throw, taking 21 (6d6) psychic damage on a failed save, or half as much damage on a successful one.