Armor Swarm

Regeneration. The swarm regains 20 hit points at the start of its turn. If the swarm takes lightning damage, this trait ceases to function until the start of the swarm's next turn. The swarm dies only if it starts its turn with 0 hit points and doesn't regenerate.

Whirling Weapons. Melee Weapon Attack: +8 to hit, 1 creature in the swarm's space. Hit: 13 (2d10+2) slashing damage plus 13 (2d10+2) bludgeoning damage. 

Volley (Recharge 6). The swarm unleashes a volley of arrows and crossbow bolts from its many weapons. Each creature in either a 200-foot sphere centered on the swarm, or a 120- foot cone (swarm's choice) must succeed on a DC 17 Dexterity saving throw, taking 23 (6d8) piercing damage on a failed save, or half as much damage on a successful one.

Engulf. The swarm moves up to its speed. While doing so, it can enter creatures' spaces that are as large as or smaller than the swarm. Whenever the swarm enters a creature's space, the creature must make a DC 17 Dexterity saving throw.
On a successful save, the creature can choose to be pushed 5 feet back or to the side of the swarm. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.
On a failed save, the swarm enters the creature's space, and the creature takes 6 (1d10+1) bludgeoning damage plus 6 (1d10+1) piercing damage and is engulfed. The engulfed creature is restrained and takes this damage again at the start of each of the swarm's turns. When the swarm moves, the engulfed creature moves with it.
An engulfed creature can try to escape by taking an action to make a DC 17 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the swarm. The swarm can restrain one creature that is the same size as it, two creatures one size smaller than it, or four creatures two or more sizes smaller than it.

Split. The swarm splits into two new swarms, provided it has at least 20 hit points. Roll initiative for the new swarms, which have their own turns. Skip the new swarms' turns on the round this action is used. Each new swarm has hit points equal to half the hit points of the original swarm, rounded down. New swarms are one size smaller than the original swarm. Engulfed creatures move into one of the new swarms (swarm's choice) if they are of a small enough size to be supported by the new swarm.