Death Butterfly Swarm

The death butterfly swarm's poison affects corporeal undead who are otherwise immune to poison.

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 hit points or gain temporary hit point.

A creature in a space occupied by the death butterfly swarm has its speed reduced by half, and must succeed on a DC 13 Dexterity saving throw or become blinded. Both effects end when the creature doesn't share a space with the swarm at the end of the creature's turn. If a creature succeeds on the saving throw, it is immune to the swarm's blindness (but not the speed reduction) for 24 hours.

The swarm makes a Bite attack against every target in its spaces.

Melee Weapon Attack: +3 to hit, reach 0 ft., every target in the swarm's space. Hit: 22 (6d6 +1) piercing damage, or 11 (3d6 + 1) piercing damage if the swarm has half of its hit points or fewer. The target also takes 10 (3d6) poison damage and becomes poisoned for 1d4 rounds; a successful DC 13 Constitution saving throw reduces poison damage by half and prevents the poisoned condition..