Vampire Lord, Ravenloft

Shapechanger. If the Vampire isn’t in running water or sunlight, they can use their action to polymorph into  a Medium cloud of mist, or back into their true form.

While in mist form, the Vampire can’t take any actions, speak, or manipulate objects. They are 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 he can’t pass through water. They have advantage on Strength, Dexterity, and Constitution saving throws, and are immune to all nonmagical damage, except the damage they take from sunlight.

Legendary Resistance (3/Day). If they fail a saving throw, they can choose to succeed instead.

Misty Escape. When the Vampire drops to 0 hit points outside their coffin, they transform into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious. This occurs even in sunlight or running water. They do not regenerate, cannot be destroyed, and can take no action except to move towards their coffin. At their coffin, they revert to their vampire form, begin to regenerate, and are paralyzed until an hour passes. Unlike a normal vampire, the Vampire Lord regenerates even while paralyzed.

Regeneration. They regain 20 hit points at the start of their turn if they have at least 1 hit point and aren't in running water or sunlight. If they take radiant damage, this trait doesn’t function at the start of his next turn.

Spellcasting. The Vampire Lord  is a 9th-level spellcaster. Their spellcasting ability is Intelligence (spell save DC 17, +10 to hit with spell attacks). The Vampire Lord has the following wizard spells prepared:

Cantrips (at will): mage hand, prestidigitation, ray of frost

1st level (4 slots):Zephyr Strike, shield, sleep

2nd level (3 slots): detect thoughts, gust of wind, mirror image

3rd level (3 slots): animate dead, fireball, nondetection

4th level (3 slots): Summon Greater Demon, greater invisibility, polymorph

5th level (1 slot): animate objects, scrying, infernal calling

Vampire Weaknesses. The Vampire Lord has the following flaws:

Ancient: The Vampire Lord is old beyond reckoning. Long rests mean something different to this creature, and it cannot regain spell slots during this adventure.

Stake to the Heart. If a piercing weapon made of wood is driven into their heart while they are incapacitated in his coffin, they are paralyzed until the stake is removed even if regeneration is completed.

Sunlight Hypersensitivity. While in sunlight, The Vampire Lord takes 20 radiant damage at the start of their turn, and has disadvantage on attack rolls and ability checks.

Running Water. While immersed in running water, The Vampire Lord takes 20 radiant damage at the start of their turn, and has disadvantage on attack rolls and ability checks.

Multiattack. The Vampire Lord makes two attacks, only one of which can be a bite attack.

Unarmed Strike. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) bludgeoning damage plus 14 (4d6) necrotic damage. If the target is a creature, they can grapple it (escape DC 18) instead of dealing the bludgeoning damage.

Greatsword, Strahd Only. Strahd may conjure his cursed sword as a free action:
Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) magical slashing damage plus 14 (4d6) necrotic damage. This trades grappling for direct damage.

Cestus, Irena Only. Irena may replace any unarmed attack with the following longer-ranged attack:
Little Mothers. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 8 (1d8 + 4) magical piercing damage plus 14 (4d6) necrotic damage. If the target is a creature, the Vampire Lord can grapple it (escape DC 18) instead of dealing the piercing damage. The Vampire Lord may immediately move up to half their speed towards the victim.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the Vampire Lord, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Vampire Lord regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced 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 Lord's control.

Charm. The Vampire Lord may expend a spell slot to make a psychic assault on any creature. This attack deals 2d6+4 psychic damage, +1d6 additional damage for each spell level above first. That creature must make a DC 16 Intelligence or Charisma saving throw, their choice. This counts as a saving throw against a spell.

• On a successful save, the target takes half damage and this effect cannot reduce them below 1 HP.
• On a failed save, the target's speed drops to 0 and they are unable to speak, until the end of their next turn This includes verbal components for spells.
• If this ability reduces a target to zero, instead of dying, they immediately become a vampire spawn under the Vampire Lord's control. This change is not obvious.

The Vampire Lord uses Shield aggressively once the Heart of Sorrow is disabled or destroyed, and does not seek to conserve spell slots.

The Vampire Lord 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. They regains spent legendary actions at the start of his turn.

Move. Vampire Lord moves up to their speed without provoking opportunity attacks.

Unarmed Strike. Vampire Lord makes one unarmed strike. In the case of Irena, she may replace this with a Cestus strike.

Bite (Costs 2 Actions). Vampire Lord makes one bite attack.

Charm (Costs 3 Actions). Vampire Lord makes one charm attack.