Brood of Bones

Harvest of Bones: If the swarm has less than full hit points at the end of a turn in which it damages a target, it regains hit points equal to the damage it dealt.

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 mouse.

Corrupted Bites. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 12 (2d8) necrotic damage, or 5 (1d8) necrotic damage if the swarm has half of its hit points or fewer. This necrotic damage ignores resistances. 

Scatter the Brood. If the Brood is targeted by any Spell or attack that deals damage to an area, they may use their reaction to scatter and avoid the damage, entirely on a successful save, and half damage on a failed save. The Brood then takes 1d4 rounds to reform, during which it cannot attack.