Huge Beast (Swarm), chaotic evil
Armor Class 15 (natural armor)
Hit Points 84 (13d12)
Speed fly +40, walk +5, hover true
Challenge 6
Damage Vulnerabilities cold, fire
Damage Resistances bludgeoning, piercing, and slashing
Condition Immunities charmed, frightened, paralyzed, petrified, prone, restrained, petrified
Senses darkvision 60 ft., passive Perception 12
Potent Poison. The death butterfly swarm's poison affects corporeal undead who are otherwise immune to 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 hit points or gain temporary hit points.
Weight of Wings. As death butterfly swarm but with DC 16 Dexterity saving throw
Multiattack. The swarm makes a Bite attack against every target in its spaces.
Bites. Melee Weapon Attack: +6 to hit, reach 0 ft., every target in the swarm's space. Hit: 24 (6d6 + 3) piercing damage, or 13 (3d6 + 3) piercing damage if the swarm has half of its hit points or fewer. The target also takes 17 (5d6) poison damage and becomes poisoned for 1d4 rounds; a successful DC 15 Constitution saving throw reduces poison damage by half and prevents the poisoned condition.
Source: Tome of Beasts · OGL/Open