Coin Whirlwind. The treasure horde has a shield of flying coins surrounding it. It has a +2 to AC against ranged attacks, and any creature that moves within 5 feet of the treasure horde or starts its turn there must succeed on a DC 14 Dexterity saving throw or take 4 (1d8) bludgeoning damage, taking half as much damage on a successful save.

Swarm. The treasure horde can occupy another creature's space and vice versa, and the treasure horde can move through any space large enough for a Tiny coin. The treasure horde can't gain temporary hit points. The treasure horde is difficult terrain.

Multiattack. The treasure horde makes one bite attack and uses coin barrage once or uses coin barrage three times.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 38 (6d10 + 5) piercing damage. 

Coin Barrage. The treasure horde blasts coins in a 60-ft. line that is 5 ft. wide. Creatures in the line must succeed on a DC 16 Dexterity saving throw or take 9 (2d8) bludgeoning damage, fall prone, and be pushed to the end of the line, taking half as much damage and not being pushed on a successful save.

Coin Explosion (Recharge 5-6). The treasure horde blasts coins outward in a 30-ft. sphere. Creatures in the sphere must succeed on a DC 16 Dexterity saving throw or take 27 (6d8) bludgeoning damage and fall prone, taking half as much damage and not falling prone on a successful save.

Coin Prison. The treasure horde moves up to its speed. When it ends its move, any creatures in its space must succeed on a DC 16 Strength saving throw or take 22 (4d10) bludgeoning damage and become restrained, taking half as much damage and not being restrained on a successful save.
A restrained creature may use its action to escape, doing so on a successful DC 16 Athletics or Acrobatics check. In addition, another creature can use its action to pull a restrained creature out, doing so on a successful DC 16 Athletics check, rolled with advantage.
While restraining creatures, the treasure horde can't move, can't use this action, and on each of its turns, the restrained creatures must succeed on a DC 16 Constitution saving throw or take 9 (2d8) bludgeoning damage, taking half as much damage on a successful save.