Burning Rage. His anger at all living things emanates from him in waves, singing the very souls of all those near to him. (At the beginning of each turn, all within 20 feet of him make a wisdom save DC 19. On a failed save, take 3d12 fire damage, on a successful save, take half. Ignore any advantages.)

Spellcasting. Zyrroth is a 9th-level spellcaster. His spellcasting ability is Charisma (spell save DC 13 + 5 to hit with spell attacks). Zyrroth has the following Wizard spells prepared:
Cantrips (at will): Control Flames, Create Bonfire, Fire Bolt, Green-Flame Blade, True Strike, Prestidigitation
1st level (5 slots): Burning Hands, Chromatic Orb, Cause Fear, False Life, Witch Bolt

2nd level (4 slots): Aganazzar's Scorcher, Continual Flame, Darkness, Flaming Sphere

3rd level (4 slots): Dispel Magic, Fireball, Melf's Minute Meteor, Incite Greed

4th level (4 slots): Fire Shield, Greater Invisibility,  Wall of Fire

5th level (4 slots): Contact Other Plane, Immolation, Conjure Elemental, Passwall

6th level (3 slots): Contingency, Disintegrate, Globe of Invulnerability

7th level (3 slots): Delayed Blast Fireball, Finger of Death, Plane Shift

8th level (2 slots): Maze, Sunburst, Incendiary Cloud

9th level (2 slots): Meteor Swarm, Time Stop

Chains of Flame.  Any Target within 10 feet of Zyrroth must make a dexterity save (DC 18), on a failed save they are immobilized for at least 1 turn, after which they may make a strength save (DC 20) to escape the chains. For each turn the target is bound by the flames, they take 5d10 fire damage.

Action Flame Scythe. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit:  20 + (4d6 + 10) fire damage. 

Action Frost Scepter. Ranged Weapon Attack: +7 to hit, range 40/120 ft., 1 target. Hit: 15 +(4d4 + 8) freezing damage.

Cold Judgement. Zyrroth lets loose a bone-chilling howl, target takes 10d10 Freezing damage.

Legends tell of a demon lord with the power to destroy the world, you happen to have had the misfortune of engaging that very same demon lord, good luck.

Ragnarok (This action requires Zyrroth to have no remaining spell slots, and gives him 4 exhaustion points after usage, takes 4 complete turns of inaction to prepare). For the 4 turns prior to this action being used, Zyrroth will grow in both size and heat, eventually getting too hot for anything to approach, let alone you flimsy mortals. He will eventually grow to the size of a Colossal+ creature, going up one size level for each round of the charge up. The aura of heat has a radius equal to one Zyrroth in his present state. All within the Aura of Heat will be unable to move towards Zyrroth or take any action against him, they will also take 5d20+30 fire damage per turn within the globe of heat. Any projectiles fired towards Zyrroth from outside the globe of heat will burn up before ever reaching him, and any preexisting magic within the globe of heat will burn up immediately. Once the charge up is complete, a blinding light will engulf the entire planet and Zyrroth will return to his original size. 7 days after the usage of this action, A rain of hell fire will engulf the entire planet, remaining indefinitely until every last living cell is charred to a crisp. The fire will then dissipate, leaving only the sad, desolated remains of a once vibrant planet.