Swarm of Sunflies

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

Melee Weapon Attack +5 to hit, reach 0 ft., one creature in the swarm's space. 10 (3d4 + 3) Piercing damage, or 5 (1d4 + 3) Piercing damage if the swarm has half of its hit points or fewer. Additionally, if the swarm is on an Outer Plane, it injects the target with a toxin, the effect of which is determined by the swarm's location:

The target sheds bright light in a 5-foot radius until the end of its next turn. During that time, the Invisible condition has no effect on it.

If the target is Concentrating on a spell or similar effect, it loses its Concentration.

The target's speed is reduced by 10 feet until the end of its next turn.

The swarm shines its lights in a dazzling display. Each creature within 15 feet of the swarm must succeed on a DC 10 Constitution saving throw or have the Stunned condition for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The swarm sheds bright light in a 15-foot radius and dim light for an additional 15 feet, or it uses a bonus action to extinguish the light.