Large swarm of tiny undead, chaotic evil

Armor Class 17 (natural armor)
Hit Points 198 (36d10)
Speed 20 ft., fly 60 ft.

Saving Throws Dex +8, Wis +6, Cha +9
Skills Acrobatics +8, Perception +6, Stealth +8
Damage Vulnerabilities bludgeoning
Damage Resistances piercing and slashing from nonmagical weapons
Damage Immunities poison
Condition Immunities charmed, exhaustion, frightened, paralyzed, poisoned, prone, restrained, stunned
Senses darkvision 60 ft., passive Perception 16
Languages Common, Void Speech
Challenge 10 (5,900 XP)

Special Traits

• Strength of Bone. A bone swarm can choose to deal bludgeoning, piercing, or slashing damage, and adds 1.5× its Strength bonus on swarm damage rolls as bits and pieces of broken skeletons claw, bite, stab, and slam at the victim.

Actions

• Multiattack. The bone swarm can attack every hostile creature in its space with swirling bones.
• Swirling Bones. Melee Weapon Attack: +10 to hit, reach 0 ft., one creature in the swarm’s space. Hit: 31 (5d8 + 9) bludgeoning, piercing, or slashing damage (includes Strength of Bone special ability).
• Death’s Embrace (Recharge 5-6). Melee Weapon Attack: +10 to hit, reach 0 ft., one creature in the swarm’s space. Hit: the target is grappled (escape DC 16) and enveloped within the swarm’s bones. The swarm can force the creature to move at its normal speed wherever the bone swarm wishes. Any non-area attack against the bone swarm has a 50% chance of hitting a creature grappled in Death’s Embrace instead.
• Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a human skull. The swarm can’t regain hit points or gain temporary hit points.

About

Dank winds sweep up skeletons, both humanoid and animal. They blow forward, reaching out for living creatures like a clawed hand of bone. A scattering of bones rolls across the ground, then rises into the air, billowing like a sheet.

Swarms of Fallen. On rare occasions, the pugnacious spirits of fallen undead join together, bonded by a common craving: to feel alive again. They gather up their bones from life, as well as any other bones they come across, and form bone swarms.

Nomadic Undead. These swarms then ravage the countryside wresting life from living creatures, grabbing livestock, humanoids, and even dragons, digging in their claws in an attempt to cling to life. Bone swarms with one or more sets of jaws wail constantly in their sorrow, interrupting their cries with snippets of rational but scattered speech declaiming their woes and despair.

Cliff and Pit Dwellers. Bone swarms gather near cliffs, crevasses, and pits in the hope of forcing a victim or an entire herd of animals to fall to its death, creating more shattered bones to add to their mass.