Arvon

Dangerous at both ends and crafty in the middle. Arvon cannot be flanked by enemies while conscious 

Spellcasting. Arvon is a 19-level spellcaster. Its spellcasting ability is 18 (spell save DC 18, +10 to hit with spell attacks). Arvon has following spells prepared:
Cantrips (at will): detect magic, see invisibility, wall of ice

2/ per day: dispel magic, compelled duel
1/ per day: power word kill, power word stun

Multiattack. Arvon makes four Greatsword attacks.

Greatsword. Melee Weapon Attack: +20 to hit, reach 10 ft., one target. Hit: 9 (4d8 + 10) slashing damage, or 10 (4d10 + 15) slashing damage if used with two hands, plus 18 (6d8) necrotic damage.

Great Weapon Fighting: When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll. The weapon must have the two-handed or versatile property for you to gain this benefit.

Hellfire Orb (1/Day). Arvon hurls 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. The sphere spreads around corners. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Magic Resistance. Arvon has advantage on saving throws against spells and other magical effects and if Arvon rolls a natural 19 or 20, then the spell is reflected against the caster.

Marshal Undead. Unless Arvon is incapacitated, it and undead creatures of its choice within 60 feet of it have advantage on saving throws against features that turn undead.

Spellcasting. Arvon is a 19th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following paladin spells prepared:

1st level (4 slots): command, compelled duel. searing smite

2nd level (3 slots): hold person, magic weapon

3rd level (3 slots): dispel magic, elemental weapon

4th level (3 slots): banishment, staggering smite

5th level (2 slots): destructive wave (necrotic)

Unholy Smite. When you hit a creature with a melee weapon attack, you can expend one spell slot to deal necrotic damage to the target, in addition to the weapon’s damage. The extra damage is 2d8 for a 1st-level spell slot, plus 1d8 for each spell level higher than 1st, to a maximum of 5d8. The damage increases by 1d8 if the target is of good alignment.

Fear Aura . Any creature that starts its turn within 5 feet of Arvon must make a DC 20 Wisdom saving throw. On a failed save, the creature is frightened until the start of its next turn. If a creature’s saving throw is successful, the creature is immune to Arvon's Fear Aura for the next 24 hours.

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

Arvon 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. Arvon regains spent legendary actions at the end of his turn.

Legendary resistence (Costs 1 Actions). If Arvon fails a saving throw or ability check on another players turn, he can choose to succeed insetad

Attack (Costs 1 actions) Arvon can make 1 melee attack against a creature within range