Swarm of Prismatic Beetles

Glittering Carapace. When a creature that can see the swarm starts its turn within 30 feet of the swarm and the swarm is in bright or dim light, the swarm can force the creature to make a DC 13 Wisdom saving throw if the swarm isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature falls unconscious. Otherwise, a creature that fails the save is incapacitated and its speed is reduced to 0 until the start of its next turn, as it remains transfixed in place by the glitter of the swarm. A creature has advantage on the saving throw if the swarm is in dim light.

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

Bites. Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm's space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hp or fewer. The target must make a DC 13 Constitution saving throw. On a failure, a creature takes 10 (4d4) poison damage and is euphoric for 1 minute. On a success, a creature takes half the damage and isn't euphoric. A euphoric creature has disadvantage on saving throws. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.