Champion of the Hells

Magic Resistance. The Champion of the Hells has advantage on saving throws against spells and other magical effects.

Marshal Fiends. Unless the Champion of the Hells is incapacitated, they and fiends of their choice within 60 feet of them have advantage on saving throws against features that turn fiends.

Spellcasting. The Champion of the Hells is a 19th-level spellcaster. Their spellcasting ability is CHA (spell save DC 18, +10 to hit with spell attacks). The Champion of the Hells has following paladin spells prepared: 
1st level (4 slots): command, compelled duel, searing smite
2nd level (3 slots): hold person, locate object 
3rd level (3 slots): dispel magic, counterspell 
4th level (3 slots): banishment, staggering smite 
5th level (2 slots): destructive wave (necrotic)

Multiattack. The Champion of the Hells makes three attacks with the Mace of the Black Crown.

Mace of the Black Crown. Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target. Hit: 10 (1d6 + 8) bludgeoning damage plus 8 (3d6) fire damage. 

Hellfire (1/Day). The Champion of the Hells creates and throws a magical ball of fire that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot radius sphere centered on that point must make a DC 18 Dexterity saving throw. A creature takes 55 (14d6) points of fire damage on a failed save, or half as much on a successful one.

Summon Devil (1/Day). The Champion of the Hells summons an imp, a bearded devil or a barbed devil

Parry. The Champion of the Hells adds 6 to its AC against one melee attack that would hit it. To do so, the Champion of the Hells must see the attacker and be wielding a melee weapon.

Rebuke (3/Day). Immediately after an attacker within 30 feet of them deals damage with an attack against a creature other than them, the Champion of the Hells forces the attacker to make a Wisdom saving throw. On a failed save, the attacker takes necrotic damage equal to the damage it just dealt. On a successful save, it takes half as much damage.

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

Attack. The Champion of the Hells makes one attack with the Mace of the Black Crown.

Command Fiend (Costs 2 Actions). The Champion of the Hells commands one allied fiend to make an attack.