Volcana, the Shard of Destruction

Skull of Volcana. An unoccupied space in the arena has the Skull of Volcana on it. This skull has a shield that whenever the players destroy all four crystals on the edges of the Arena, the shield deactivates for one round. If the party doesn't destroy the Skull in the round, the shield recharges and the crystals respawn in the respective areas they were before. The Skull has 20 HP. If the Skull reaches zero hitpoints, Volcana dies.

Immobile. Volcana cannot move, nor be moved by any effects or spells.

Multiattack. Volcana makes three attacks of its choice.

Rock Crush. Melee Weapon Attack: +16 to hit, reach 120ft., one target. Hit: 38 (5d12 + 8) bludgeoning damage. 

Fire Sphere. Custom Weapon Attack: +18 to hit, reach 120ft., multiple target. Hit: 32 (8d6 + 8) fire damage. Volcana chooses a spot. 5ft. sphere from that spot. All players in the outer ring must move 5ft. as their reaction to avoid being hit by the spot. If inside the center, the player can move 5ft. as a reaction, but they still take damage. If a player makes it out've the sphere with the 5ft. movement, they don't take any damage.

Laser Pierce. Ranged Weapon Attack: +23 to hit, reach 5/150 ft., one target. Hit: 27 (4d10 + 7) radiant damage.

Fire Swarm. Custom Weapon Attack. If Volcana uses this move, ten 5ft. spheres appear on the board. These spheres are bright light, and do not do damage if a player is in one when they form. After one round, the spheres ablaze, and all players inside of them take 50 (8d12+2) fire damage. Every time Volcana uses this, the number of spheres that can be placed increase by five.

Move Enemy. If Volcana can see an enemy on the board within 120ft. of it, it can move that player as a bonus action up to 40ft. from their current position. The creature is picked up and lifted 5ft. off the ground and moved to a spot, in which they are dropped 5ft. It cannot move any creature down or up.