Vampire Dragon

 Shapechanger. If the Dragon isn't in sun light or running water, it can use its action to polymorph into a Giant bat or a Large cloud of mist, or back into its true form. While in bat form, the dragon can't speak, its walking speed is 10 feet, and it has a flying speed of 60 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 dragon can't take any actions, speak, or manipulate objects. It is weightless, has a flying speed of 30 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.

Misty Escape. When it drops to 0 hit points outside its Lair, the Dragon transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that it isn't in sunlight or running water. If it can't transform, it is destroyed. While it has 0 hit points in mist form, it can't revert to its dragon form, and it must reach its lair within 2 hours or be destroyed. Once in its lair, it reverts to its dragon form. It is then paralyzed until it regains at least 1 hit point. After spending 1 hour in its resting place with 0 hit points, it regains 1 hit point.

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.

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

Vampire Weaknesses. 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 vampire takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

Multiattack. The dragon can use its Frightful Presence or its Charm. 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 target. Hit: 17 (2d10 + 6) piercing damage plus 8 (2d8) 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 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 dragon's control

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.  Instead of dealing damage, the dragon can grapple the target (escape DC 21)

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Charm. The dragon targets one humanoid it can see within 30 ft. of it. If the target can see the dragon, the target must succeed on a DC 16 Wisdom saving throw against this magic or be 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 bit 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 or until the dragon is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect. The dragon can only charm two people at a time, if it charms a third person the effect on the first charmed person ends.

Frightful 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 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Necrotic Breath (Recharge 5–6). The dragon exhales a deathly fog in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) necrotic damage on a failed save, or half as much damage on a successful one.

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.

Detect. The dragon makes a Wisdom (Perception) check.

Tail. The dragon makes one tail attack.

Bite (Costs 2 Actions). The dragon makes one bite 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.