Corrupt Duke

Arcane Recovery (1/Day). When he finishes a short rest, he recovers all his spell slots of 4th level and lower.

Legendary Resistance (4/Day). If he fails a saving throw, he can choose to succeed instead.

Spellcasting. He is a 18th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 18, +8 to hit with spell attacks). He can cast charm person,  misty step, Detect Thoughts, and Modify Memory at will. 

Cantrips (at will): encode thoughts, mind sliver, dancing lights, mage hand, prestidigitation

1st level (4 slots): silvery barbs, Mage Armor, shield, command

2nd level (4 slots): crown of madness, suggestion, hold person, tasha's mind whip

3rd level (3 slots): counterspell, dispel magic, enemies abound

4th level (3 slots): confusion, raulothim's psychic lance, greater invisibility

5th level (2 slots): Dominate Person, geas, Mislead

6th level (2 slot): mass suggestion, Globe of Invulnerability

7th level (1 slot): Etherealness

8th level (1 slot): Antipathy/Sympathy

Split Enchantment: When he casts an enchantment spell that targets only one creature, he can have it target a second creature. 

Alter Memories: When he casts an enchantment spell to charm one or more creatures, he can alter one creature’s understanding so that it remains unaware of being charmed.

Additionally, once before the spell expires, he can use an action to try to make the chosen creature forget some of the time it spent charmed. The creature must succeed on an Intelligence saving throw against his spell save DC or lose up to 5 hours of their memories. You can make the creature forget less time, and the amount of time can’t exceed the duration of your enchantment spell. 

Telepathic: He can speak telepathically to any creature he can see within 60 feet of you. His telepathic utterances are in a language he knows, and the creature understands you only if it knows that language. His communication doesn’t give the creature the ability to respond to you telepathically.

Slave Runes: Whenever anyone fails a saving throw from one of the Corrupt Duke’s abilities, he may have them receive an arcane collar with a maximum number of 10 runes. This collar is invisible to everyone except both the Corrupt Duke and it’s bearer unless someone makes a detect magic investigation check DC 18 and will see a vague outline of the collar and any runes present. Upon failing any saving throws from his effects, they also receive +1 rune. If  they succeed on any of his saving throws, 1 rune disappears. For each of their Slave Runes they receive a -1 penalty to any saving throws against the Corrupt Dukes effects. Slave Runes disappear one at a time every 24 hours or can be dispelled by either at least a 8th level dispel magic DC roll or the wish spell. 

Rune Shock: He taps into the Slave Runes causing each creature he chooses within 60ft of him and has any Slave Runes must succeed on a constitution saving throw against his spell DC or take 4d6 psychic damage + 3 more psychic damage for each Slave Rune they posses, or half as much on a success. 

Instinctive Charm: When a creature you can see within 30 feet of you makes an attack roll against you, you can use your reaction to divert the attack, provided that another creature is within the attack’s remaining movement and or attacking range. The attacker must make a Wisdom saving throw against your spell save DC. On a failed save, the attacker must use any remaining movement to reach the creature that is closest to it, or aim its ranged attack to the creature that is closest to it, not including the Corrupt Duke or itself. If multiple creatures are closest, the attacker chooses which one to target. If the creature cannot reach the targeted creature with its remaining movements the effect ends. Creatures that can’t be charmed are immune to this effect.

Corrupt Duke can take 4 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. He regains spent legendary actions at the start of his turn.

Rune Shock. 

Cast Spell. He can casts a spell of 3rd level or lower.

Hypnotic Gaze. Choose one creature that he can see within 5 feet of you. If the target can see or hear him, it must succeed on a Wisdom saving throw against his spell save DC or be charmed by him until the end of the targets next turn. The charmed creature’s speed drops to 0, and the creature is incapacitated and visibly dazed.

This effect ends if he moves more than 5 feet away from the creature, if the creature can neither see nor hear him, or if the creature takes damage.

The CR of the Corrupt Duke can be treated as 27. If so, when his hit points drop to 0, He transforms into his true form of a skeletal undead dragon. You can use the stat block of any ancient dragon for this transformation, resetting his HP and legendary resistances to that of the dragons. The dragon keeps all traits, actions, bonus actions, reactions, legendary actions, and lair actions, in addition to the dragons abilities. The default dragon transformation is an Ancient Emerald Dragon.