Arkhan the Cruel: Champion of Tiamat

Aura of Hate. While Arkhan isn’t incapacitated, he and all fiends and undead within 30 feet of him deal 4 extra damage whenever they hit with a melee weapon attack (already factored into Arkhan’s attacks). This extra damage is of the same type as the weapon’s damage type.

Innate Spellcasting. Arkhan’s spellcasting ability is Charisma (spell save DC 20). He can cast the following spells, requiring no material components:

3/day each: animate dead, branding smite (at 4th level), revivify

1/day each: geas, raise dead, teleport

Special Equipment. Arkhan wields a modifed Fane-Eater that was fused with Matalotok and wears a suit of Obsidian Flint Dragon Plate. The armor gives Arkhan advantage on ability checks and saving throws made to avoid or end the grappled condition on him and the weapon grants him immunity to cold damage. 

Blessing of Tiamat.  Tiamat has blessed Arkhan as her champion and bestowed him great abilities. He is proficient in all saving throws, has a +2 to AC (already  factored into stat block), resistance to Acid, Poison, Fire, Lighting, Cold, Necrotic, and Radiant (this is upgraded to immunity if Arkhan has resistance from some other source),  has immunity to the frightened, charmed, blinded, deafened,  poisoned,, petrified, paralyzed, and stunned  conditions, he has been granted a set of dragon wings which give him a 60ft fly speed, and she blesses his weapon strikes with the mighty roar of chromatic dragons giving it a bonus of 3d8 thunder damage (already shown on stat block). She has also given Arkhan Legendary Resistances and Actions. Should Arkhan loose favor with Tiamat, or Tiamat die, he immediately looses these benefits.

 Legendary Resistance (3/Day). If Arkhan fails a saving throw, she can choose to succeed instead.  

Multiattack. Arkhan makes three weapon attacks.

Fane-Eater Matalotok (Battleaxe Warhammer). Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 21 (2d8 + 12) slashing damage, or 23 (2d10 + 12) slashing damage when used with two hands, plus 13 (3d8) thunder damage. In addition, the weapon emits a burst of dragon roars that deals (3d6)  thunder damage to each creature within 30 feet of it.
If the target is a creature and Arkhan rolls a 20 on the attack roll, the creature takes an extra 13 (3d8) necrotic damage, and Arkhan regains an amount of hit points equal to the necrotic damage dealt. 

Javelin. Melee or Ranged Weapon Attack: +13 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 16 (2d6 + 9) piercing damage, plus 13 (3d8) thunder damage

Arkhan 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. The Arkhan regains spent legendary actions at the start of its turn.

Detect. Arkhan makes a Wisdom (Perception) check.

Attack. Arkhan makes one attack with Fane-Eater Matalotok.

Chromatic Flare (Costs 2 Actions). Arkhan channels his connection to Tiamat and flares with elemental energy. Each creature of the Arkhan’s choice in a 60-foot-radius sphere centered on her must make a DC 27 Dexterity saving throw. On a failed save, the creature takes 39 (6d12) acid, cold, fire, lightning, and poison damage. On a successful save, the creature takes half as much damage. To resist this damage, you mush either have resistance to all of the mentioned damage types, have immunity to three of them, or be able to absorb two of the damage types.