Cearisse Reinswaulde, Corrupted King of Man

Eldritch Rebirth (Recharges after a Short or Long Rest). If Cearisse would be reduced to 0 hit points, his current hit point total instead resets to 375 hit points, he regains spent Spell Slots, and he regains any expended uses of Legendary Resistance. Additionally, he can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Cearisse Reinswaulde, Corrupted King of Man after his Eldritch Rebirth activates.

Shapechanger. If the vampire isn't in sunlight or running water, it can use its action to polymorph into a Tiny bat or a Medium cloud of mist, or back into its true form.

While in bat form, the vampire can't speak, its walking speed is 5 feet, and it has a flying speed of 30 feet. Its statistics, other than its size and speed, are unchanged. Anything it is wearing transforms with it, but nothing it is carrying does. It reverts to its true form if it dies.

While in mist form, the vampire can't take any actions, speak, or manipulate objects. It is weightless, has a flying speed of 20 feet, can hover, and can enter a hostile creature's space and stop there. In addition, if air can pass through a space, the mist can do so without squeezing, and it can’t pass through water. It has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to all nonmagical damage, except the damage it takes from sunlight.

Legendary Resistance (3/Day). If the vampire fails a saving throw, it can choose to succeed instead.

Blood of Madness. If a creature hits Cearisse with a melee attack, they must make a DC 19 Wisdom saving throw. If the creature fails, their sanity slips away from them, and they act as though under the effects of the confusion spell.

Regeneration. Cearisse regains 25 hit points at the start of its turn if he isn't in sunlight or running water. If Cearisse takes radiant damage or damage from holy water, this trait doesn't function at the start of his next turn. If he starts his turn with 0 hit points and does not regenerate hit points, he dies.

Spider Climb. Cearisse can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Innate Spellcasting. Cearisse’s innate spellcasting ability is Charisma (spell save DC 26). It can innately cast the following spells, requiring no material components:

At will: detect magic,disguise self, mage armor

Spellcasting. Cearisse is a 19th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 26, +18 to hit with spell attacks). He regains his expended spell slots when it finishes a short or long rest. He knows the following warlock spells:

Cantrips (at will): chill touch, mage hand, minor illusion, poison spray

1st–5th level (4 5th-level slots): arms of Hadar, blight, counterspell, crown of madness, darkness, dimension door, dispel magic, fly, hold monster, invisibility

1/day: foresight, maddening darkness, crown of stars, mental prison

Multiattack. (Vampire Form Only). The vampire makes four attacks, only one of which can be a bite attack.

Unarmed Strike (Vampire Form Only). Melee Weapon Attack: +19 to hit, reach 5 ft., one creature. Hit: 14 (1d8 + 10) bludgeoning damage plus 9 (2d8) psychic damage. Instead of dealing damage, the vampire can grapple the target (escape DC 27).

Bite. (Bat or Vampire Form Only). Melee Weapon Attack: +19 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained. Hit: 13 (1d6 + 10) piercing damage plus 20 (6d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire's control.

Call the Stars. Ranged Spell Attack: +18 to hit, reach 120 ft., one to three creatures. Hit: 14 (1d10 + 9) radiant damage plus 11 (2d10) psychic damage.. 

Charm. The vampire targets one humanoid it can see within 30 feet of it. If the target can see the vampire, the target must succeed on a DC 26 Wisdom saving throw against this magic or be charmed by the vampire. The charmed target regards the vampire as a trusted friend to be heeded and protected. Although the target isn't under the vampire's control, it takes the vampire's requests or actions in the most favorable way it can, and it is a willing target for the vampire's bite attack.

Each time the vampire or the vampire's companions do anything harmful to the target, it can repeat the saving throw, ending the effect on itself on a success. Otherwise, the effect lasts 24 hours or until the vampire is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

The vampire can take 5 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 vampire regains spent legendary actions at the start of its turn.

Move. The vampire moves up to its speed without provoking opportunity attacks.

Unarmed Strike. The vampire makes one unarmed strike.

Bite (Costs 2 Actions). The vampire makes one bite attack.

If the Cearisse’s Eldritch Rebirth trait has activated in the last hour, he can use the options below as legendary actions.

Glimmering Moonlight (Costs 3 Actions). Cearisse casts the spell moonbeam on 4 points within the spells range using his spell save DC. These do not require concentration and last for 1d4+1 rounds. 

Unknowable Attack (Costs 5 Actions). Cearisse attack with his eldritch appendages in a manner that is nearly incomprehensible. All creatures of his choice within 30 ft. of him takes 8d12 bludgeoning damage and must succeed on a DC 15 Constitution saving throw or be paralyzed until the end of their next turn.