Azarr Kul

Shocking Weapons. When Azarr Kul hits a creature with any weapon, the weapon deals an extra 1d8 lightning damage (included in the weapon attack's damage below).

Legendary Resistance (3/Day). If Azarr Kul fails a saving throw, he can choose to succeed instead.

Spellcasting. Azarr Kul is a 13th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 18, +10 to hit with spell attacks). He has the following spells prepared:
Cantrips (at will): Chill Touch, Eldritch Blast, True Strike, Toll the Dead
1st level (4 slots): Command, Detect Magic, Hellish Rebuke. Inflict Wounds
2nd Level (3 slots): Blindness/Deafness, Crown of Madness, Misty Step, Shatter
3rd Level (3 slots): Counterspell, Enemies Abound, Lightning Bolt
4th Level (3 slots): Blight, Sickening Radiance, Summon Greater Demon
5th Level (2 slots): Enervation, Synaptic Static
6th Level (1 slot): Mental Prison
7th Level (1 slot): Power Word Pain 

Reaper. Whenever Azarr Kul uses a necromancy cantrip such as Chill Touch or Toll the Dead, he can target two creatures within range and within 5 feet of each other.

Multiattack. Azarr Kul can either make two attacks with his longsword when he takes the Attack action, or he can cast a spell.

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 7 (1d4 + 5) slashing damage, plus an extra 4 (1d8) lightning damage.

Blue Dragonfang Longsword. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 11 (1d10 + 6) slashing damage, plus an extra 4 (1d8) lightning damage. 

Channel Divinity: Touch of Death. When Azarr Kul hits a creature with a melee attack, he can use his Channel Divinity to deal an extra 18 points of necrotic damage to the target. He can do this twice per short or long rest.

Lightning Breath (Recharge 6). As a bonus action, Azarr Kul can shoot a bolt of lightning out in a 60-ft straight line. All creatures in the bolt's path must make a Dexterity Saving Throw against a DC 15, taking 6d8 lightning damage on a failed save or half as much on a successful save. At the beginning of his turn, roll a d6. On a 6, his lightning breath recharges.

At the end of a player's turn (a different player's turn each round), Azarr Kul can use a Legendary Action. He can only use one of these per turn.

Ferocious Warrior. Azarr Kul can move up to half his walking speed and can either cast a cantrip or make a single melee attack.

Azarr Kul's mythic abilities activate when his hit points reach specific levels. The following abilities activate depending on his hit point threshold reached at points through the battle. 

Helping Hand Into the Grave (Activates at 50 HP).  Charred, Demon-like hands break through the ground all around the players, attempting to grasp at them. Each player needs to make a DC 14 Dexterity Saving Throw. On a success, they evade the demon hands. On a failure, they are Grappled. At the start of the grappled players turn, they may attempt a DC 14 Strength (Athletics) or Dexterity (Acrobatics) Check. If they succeed, they break free. If they fail, the hands begin pulling them into the ground till they are waist-deep in it and they are now Restrained. The hands hold them in this position, and the restrained player may make a DC 16 Strength (Athletics) check to pull themselves free and out of the ground at the beginning of their next turn. Otherwise, they are restrained until they break free.