Corrupted King of Under-Mountain

Elemental Assault. Any creature that ends its turn within 5 feet of the Corrupted King takes 6 (1d10) fire damage plus 6 (1d10) cold damage, plus 13 (2d10) force damage, and needs to make a Strength save DC 17. On a fail, the creature is pushed 10 feet away from the King. 

Awkward Form. The Corrupted King has disadvantage on Strength, Dexterity, and Constitution saving throws and ability checks.

Purely Elemental. The Corrupted King has double vulnerability to any damage that is nonmagical, taking 4 times the normal damage.

Innate Spellcasting. The Corrupted King's innate spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

At will: fireball, lightning bolt, melf's minute meteors, wind wall

3/day each: erupting earth, elemental bane, fire shield, stoneskin

Spell's toll. When the Corrupted King casts erupting earth, fire shield, or stoneskin, he takes 10 (2d6 +3) magical bludgeoning damage.

Multiattack. The Corrupted King makes 3 attacks, in any combination of its Stomp and Thunder Fist attacks.

Stomp. Melee Weapon Attack: +12 to hit, reach 5 ft., any targets. Hit:  18 (2d10 + 7) thunder damage + 18 (2d10 + 7) force damage.

Thunder Fist. Ranged Weapon Attack: +12 to hit, range 30 ft., 2 targets. Hit: 20 (2d10 + 9) lightning damage.

Elemental Control (1/day). The King summons up to 3 fire elementals, 3 earth elementals, 3 water elementals, and 3 air elementals, losing 30 health for each elemental summoned in this way. Summoned elementals have maximum hit points, appear from the chaos around the boss area, and disappear if the Corrupted King is reduced to 0 hit points.

The Corrupted King can take 6 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creatures turn. The Corrupted King regains 3 legendary actions at the start of his turn, with a maximum of 6 legendary actions held at a time.

Elemental Protection. The Corrupted King becomes immune to Acid, Cold, Fire, Lightning, or Thunder damage until the end of the round. When the Corrupted King uses this ability for the second time in a round, the King loses immunity to the first damage type unless he expends an additional legendary action for each additional time he has used this ability this round. (1st time : 1 point; 2nd time : 2 points; 3rd time : 3 points).

Stone Meld (Costs 2 Actions). The Corrupted King melds into the stone beneath him, provoking opportunity attacks from creatures in range, and reappears, taking form up to 120 feet away in an unoccupied space.

Snatch (Costs 2 Actions). The Corrupted King reaches out and grabs an enemy within 10 feet of him. The targeted creature makes a Strength save DC 21. On a fail, the creature takes 23 (3d8 + 9) and is grappled by the Corrupted King. On a success, the creature takes half damage and is not grappled. When the Corrupted King moves, creatures grappled in this way move with him. Creatures grappled in this way can make a DC 21 Strength save at the beginning of their turn to free themselves, and take 4 (1d6) bludgeoning damage from falling to the ground.

Elemental Storm (Costs 6 Actions). The Corrupted King summons the elemental chaos around him, slamming it into the arena. Every creature in the area takes 10 (2d6 + 3) damage of each magic damage type. (Acid, Cold, Fire, Force, Lightning, Necrotic, Poison, Psychic, Radiant, and Thunder). When the Corrupted King takes damage in this way, it ignores his resistances and immunities.