Shapechanger. If the dragon isn't in sun light or running water, it can use its action to polymorph into a Large cloud of mist, or back into its true form.
While in mist form, the dragon 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 dragon fails a saving throw, it can choose to succeed instead.
Regeneration. The dragon regains 20 hit points at the start of its turn if it has at least 1 hit point and isn't in sunlight or running water. If the dragon takes radiant damage or damage from holy water, this trait doesn't function at the start of the dragon's next turn.
Vampire Weakness. The dragon has the following flaws.
Harmed By Running Water. The dragon takes 20 acid damage if it ends its turn in running water.
Stake to the Heart. If a piercing weapon made of wood is driven into the dragon's heart while the dragon is incapacitated in its resting place, the dragon is paralyzed until the stake is removed.
Sunlight Hypersensitivity. The dragon takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.
Wand of Orcus. Holy water within 10 feet of the dragon is destroyed. Creatures cannot benefit from short or long rests while within 300 feet of the wand.
Multiattack. The dragon can use its Charming Presence. It then makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained. Hit: 8 (1d8 + 4) piercing damage plus 13 (3d8) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the dragon regains hit points equal to the amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.
Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.
Bestow Curse. While wielding the wand of Orcus, the dragon can use its action to cast bestow curse.
Inflict Wounds. While wielding the wand of Orcus, the dragon can use its action to cast inflict wounds.
Charming Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 17 Wisdom saving throw or become charmed by the dragon. The charmed target regards the dragon as a trusted friend to be heeded and protected. Although the target isn't under the dragon's control, it takes the dragon's requests or actions in the most favorable way it can, and it is a willing target for the dragon's bite attack.
Each time the dragon or the dragon'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 long or until the dragon is destroyed, is on a different plane or existence than the target, or takes a bonus action to end the effect.
Necrotic Breath (Recharge 5–6). The dragon exhales an deathly cold blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8) necrotic damage on a failed save, or half as much damage on a successful one. Creatures damaged by this attack cannot regain hit points until the start of the dragon's next turn.
In addition, 2d4 mist spawn (shadows MM) appear in the area of effect, and immediately act on the dragon's initiative.
The dragon 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. The dragon regains spent legendary actions at the start of its turn.
Move. The dragon moves up to its speed without provoking opportunity attacks.
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Bite (Costs 2 Actions). The dragon makes one bite attack.