Al' Diabolos, the Lord of Terror

The True Prime Evil. If Al' Diabolos, the Lord of Terror would be reduced to 0 hit points, its current hit point total instead resets to 450 hit points, and it recharges its Infernal Screech and he regains any expended uses of Legendary Resistance. Additionally, Al' Diabolos, the Lord of Terror can now use the options in the "Mythic Actions" section for 1 hour. And all creatures currently in combat against Al' Diabolos the Lord of Terror and himself teleport to his lair if not already there.

Spellcasting. Al' Diabolos, the Lord of Terror is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 25, +18 to hit with spell attacks). Al' Diabolos, the Lord of Terror has following wizard spells prepared:
Cantrips (at will): fire bolt, toll the dead,lightning lure, shocking grasp, thunderclap

1st level (4 slots): burning hands, absorb elements, silvery barbs, shield

2nd level (3 slots): blur, darkness, scorching ray, misty step

3rd level (3 slots): fireball, summon lesser demons, Ashardalon's Stride

4th level (3 slots): banishment, greater invisibility, wall of fire, gravity sinkhole

5th level (3 slots): infernal calling, far step, conjure elemental, immolation

6th level (2 slot): summon fiend, chain lightning, disintegrate

7th level (2 slot): power word pain, reverse gravity, finger of death

8th level (1 slot): maddening darkness, dark star

9th level (1 slot): meteor swarm, psychic scream

Lord of Hell. Any spell that Al' Diabolos, the Lord of Terror casts that summons a creature only takes 1 action.

Legendary Resistance (3/Day). If Al' Diabolos, the Lord of Terror fails a saving throw, he can choose to succeed instead.

Magic Resistance. Al' Diabolos, the Lord of Terror has advantage on saving throws against spells and other magical effects.

Unholy Strikes: Al' Diabolos, the Lord of Terror's attacks are considered magical.

Demonic Tread. Al' Diabolos the Lord of Terror is unaffected by rough terrain. 

Multi-Attack. Al' Diabolos makes two attacks with his Claws, one with his Tail, and one with his Horns. Or one 4th level spell or lower and two tail attacks. 

Claw. Melee Weapon Attack: +19 to hit reach 10 ft., one target. Hit: 17 (4d8 + 9) slashing damage plus 16 (3d10) fire damage plus 16 (3d10) necrotic damage.

Tail. Melee Weapon Attack: +19 to hit reach 20 ft., one target. Hit: 13 (6d4 + 9) piercing damage plus 19 (3d12) lightning damage.

Horns. Melee Weapon Attack: +19 to hit reach 5 ft., one target. Hit: 25 (5d12 + 9) piercing damage plus 17 (4d8) fire damage.

Devilish Heave. Immediately after he hits a creature with Horns attack as a part of that same action, he can attempt to shove that target with his horns. The target must be within 5 feet of him. It must succeed on a DC 25 Strength saving throw on a failed save the target is thrown up to 20 feet away.

Infernal Screech (1/per day). Al' Diabolos, the Lord of Terror takes in a deep breath extinguishing all flames within 10ft of him and lets out an unholy roar. All creatures within 50ft of Al' Diabolos, the Lord of Terror makes a DC 25 Strength saving throw, and roll 1d10. On a successful save a creature is thrown back feet equal to the number rolled on the d10 times 10. On a failed save a creature is thrown back equal to twice the number rolled on the d10 times 10 and take 1d10 force damage per 10ft. 

Freed Will. Al' Diabolos, the Lord of Terror can can cast creature spells without having the concentration requirement on that spell at the cost of the creature(s) being summoned with half of their hp. He can also break concentration on any creature spell he has currently cast to remove the concentration requirement on that spell at the cost of those creature(s) losing half of their current hp.

Rising Hell. Al' Diabolos, the Lord of Terror can smash the ground underneath him causing the ground in a 5ft radius around and under him to become rough terrain and turns into magma and deals (4d4) fire damage to those who start their turn there or enter it during their turn. The magma cools down and becomes rough terrain in 1d4 rounds. The grounds settles and becomes regular terrain in 2d4 rounds. 

Al' Diabolos, the Lord of Terror can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Al' Diabolos, the Lord of Terror regains spent legendary actions at the start of his turn.

Cantrip. Al' Diabolos, the Lord of Terror can make an Attack using a Cantrip.

Tail (Costs 2 Actions).  Al' Diabolos, the Lord of Terror can make a Tail Attack.

Volcanic Breath (Costs 3 Actions). Al' Diabolos, the Lord of Terror lets out a explosive scream letting in a 75ft cone of hellfire. Each creature in that area must make a DC 25 Dexterity saving throw on a fail save a creature takes (10d8) fire damage or half as much on a successful save.

If Al' Diabolos, the Lord of Terror The True Prime Evil trait has activated in the last hour, he can use the options below as legendary actions.

Claw. Al' Diabolos the Lord of Terror can make a Claw Attack.

Consume the Light (Costs 2 Actions). Al' Diabolos, the Lord of Terror attempts to grapple a creature within reach. If he succeeds that creature gets teleported to a plane of pure darkness taking (10d10) psychic damage. The creature returns to the spot it was grappled from at the start of it's next turn. 

Personal Labyrinth (Costs 3 Actions). Al' Diabolos, the Lord of Terror casts  Maze.