Crowley the Infiltrator

Legendary Resistance. Crowley has 5 Legendary Resistances.

Necrotic Lose. Once per round at the cost of one spellslot, Crowley adds 2d8 Necrotic damage. This is increased by 1d8 whenever he burns a spellslot he burns above 1st.

Magic's Bane. When a creature within 15 ft. of Crowley casts a spell, he can use his reaction to make a melee weapon attack against that creature, and when he damages a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration. The Soul Slayer also have advantage on saving throws against spells cast by creatures within 5 ft. of you.

Spellcasting. Crowley is a 18th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 19, +10 to hit with spell attacks).

Crowley has following paladin spells prepared:

1st level (8 slots): Armor of Agathys, Bless, Command, Cure Wounds, Shield of Faith, Wrathful Smite

2nd level (6 slots): Hold Person, Spiritual Weapon

3rd level (6 slots): Bestow Curse, Dispel Magic, Fear

4th level (4 slots): Banishment, Dominate Beast, Staggering Smite, Stoneskin

Multiattack. Crowley attacks three times: 2 melee and 1 ranged.

The Great Deception. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 17 (2d12 + 11) slashing damage and 6 (2d6) psychic damage.

The Floating Lies. RangedWeapon Attack: +14 to hit, range 10/120 ft., 1 target. Hit: 19 (3d6 + 10) piercing damage and 6 (2d6) psychic damage.

Crowley has 4 Legendary actions.

The Lord of Nightmares (1). Crowley touches a creature within five feet of himself. That creature must make a DC 18 Wisdom saving throw. On a failure, they take 18 (6d6) psychic damage, and are transported to a realm of darkness. This realm has something that haunts the creature within it. The creature has 15 health, but the affected creature takes a third of the damage they deal upon their return.

The Screams of Faithless (2) Crowley raises his blade, opening a portal to a realm of punishment. All creatures within 30 feet must make a DC 18 Constitution saving throw, or take 36 (12d6) thunder damage.

The Eternally Faithful (3) Crowley engulfs himself in necrotic energy. For five rounds, he takes 15 necrotic damage, but he deals 25 (5d10) necrotic damage to every creature within 10 feet.