Death Butterfly Swarm

Potent Poison. Undead with resistance or immunity to poison damage or the poisoned condition can be affected by the swarm's poison.

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.

Weight of Wings. A creature that starts its turn in the swarm's space has its speed halved and must succeed on a DC 12 Dexterity saving throw or become blinded. Both effects end when the creature ends its turn outside the swarm's space. On a successful saving throw, the creature is immune to the blinding effect of Weight of Wings for the next 24 hours.

Bites. Melee Weapon Attack: +4 to hit, reach 0 ft., one creature in the swarm's space. Hit: 21 (6d6) piercing damage, or 10 (3d6) piercing damage if the swarm has half of its hp or fewer. The target must make a DC 12 Constitution saving throw. On a failure, it takes 10 (3d6) poison damage and is poisoned until the end of its next turn. On a success, it takes half the damage and isn't poisoned.