Bone Swarm

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 human skull. The swarm can't regain hp or gain temporary hp.

Undead Nature. The bone swarm doesn't require air, food, drink, or sleep.

Multiattack. The bone swarm makes two Swirling Bones attacks. It can replace one Swirling Bones attack with one Death's Embrace attack, if available.

Swirling Bones. Melee Weapon Attack: +10 to hit, reach 0 ft., one creature in the swarm's space. Hit: 36 (8d8) bludgeoning, piercing, or slashing damage (the bone swarm's choice), or 18 (4d8) bludgeoning, piercing, or slashing damage (the bone swarm's choice) if the swarm has half of its hp or fewer.

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) if it is a Large or smaller creature. Until this grapple ends, the creature is restrained and any attack against the bone swarm has a 50 percent chance of hitting the grappled creature instead. The bone swarm can have only one creature grappled at a time. The grappled creature has advantage on ability checks made to escape the grapple when the bone swarm has half of its hp or fewer.