Avatar of Mephistopheles

Devil's sight. Magical darkness does not impede the avatar's Darkvision

Magic Resistance. The Avatar has advantage on saving throws against spells and other magical effects.

Oathbreaker's Aura. The Avatar is surrounded by a 30 foot aura of Hatred. The Avatar and its allies within 30 feet of it gain a +4 bonus to damage rolls and saving throws.

Spellcasting. The Avatar is a 13th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). The Avatar has following Paladin spells prepared:
1st level (4 slots): Command,  Divine Smite, Hellish Rebuke

2st level (3 slots): Crown of Madness, Darkness, Find Steed, Zone of Truth

3st level (3 slots): Animate Dead, Fireball

4st level (1 slot): Blight, Confusion

Multiattack. The Avatar makes three Attacks. The avatar can replace one of these attacks with a use of Spellcasting or Dreadful Aspect

Hellfire Halberd. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 11(1d10 + 6) Slashing damage, 4(1d8) Fire Damage and 4(1d8) Necrotic damage.

Undead Spark. Ranged Spell Attack: +9 to hit, range 60/120 ft., 1 target. Hit: 11 (2d10) Necrotic damage.

Dreadful Aspect (3/day). Each creature of the Avatar's choice within 30 feet must make a Wisdom Saving throw (save DC 17). On a failed save,  creatures effected by this are frightened of you until the end of their next turn.

Unholy Healing (3/day). The Avatar regains 15 hit points and all creatures within 10 feet of the Avatar take 1d8 necrotic damage.

The Avatar 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 Avatar regains spent legendary actions at the start of her turn.

Fly (Costs 1 Action). The avatar flies up to its fly speed

Attack (Costs 1 Action). The avatar makes one Hellfire Halberd or Undead Spark attack

Spellcasting (Costs 2 Actions). The avatar uses Spellcasting