Vampire Dragon

Shapechanger. If the Vampire Dragon isn't in direct sunlight or running water, it can use its action to polymorph into its medium humanoid form or its medium mist form, or back to its true form. 

While in humanoid form, the Vampire dragon shares the basic abilities of the Vampire.

While in mist form, the dragon can't perform any actions, speak, or manipulate objects. It is weightless, has a flying speed of 20ft, 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, but can't go past 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 Resistence (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 resting place, the dragon transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that it isn't in direct sunlight. If it can't transform, it is destroyed.

While it has 0 hit points in mist form, it can't revert to its vampire form, and it must reach its resting place within 2 hours or be destroyed. Once in its resting place, it reverts to its vampire 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 35 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 vampire's next turn.

Vampire Weaknesses. The Dragon has the following flaws:

Forbiddance. The Dragon can't enter a residence without an invitation from one of the occupants.

Running water. The Dragon takes 20 acid damage when encountering running water. 

Stake to the Heart. If a piercing weapon made of wood is driven into the Dragon heart while the Dragon is incapacitated in its resting place, the vampire 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.

Multiattack. The Dragon can use it's Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack. +15 to hit, reach 10 ft. one target. Hit: 17 (3d6 +6) piercing damage plus 14(3d6 + 4) Necrotic damage. The targets hit points are reduced by an amount equal to the necrotic damage taken, and the dragon regains that many hit points. The reductions lasts until the target finishes a long rest. The target dies if this effect reduces their maximum hit points down to zero. 

Claws. Melee Weapon Attack. +13 to hit, reach 5 ft. one target. Hit: 14 (3d6 +4) slashing damage

Tail. Melee Weapon Attack. +14 to hit, reach 15ft, one target. Hit: 16 (4d6 +4) bludgeoning damage.

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. 

Shadow Breath (recharge 5-6 turns). The dragon exhales black shadows in a 60-foot cone. Each creature in the area must make a DC 23 constitution saving throw, Taking 70 (18d6) Necrotic damage on a failed save, or half as much damages on a successful one. Each creature that took damage this way has their maximum hit points reduced by the amount of necrotic damage they took. 

Hypnosis. The dragon targets one creature it can see within 60ft of it. If the target can see the dragon, the target must succeed on a DC 22 Wisdom saving throw against this magic or be under the Dragon's control. The controlled target see's the dragon as a trusted friend and will fight for and protect it at all costs.

Each time the dragon or dragon's companions do anything harmful to the target, it can repeat the saving throw, ending the effect on a success. Otherwise, the effect lasts for 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 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 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 20 Dexterity saving throw or take 20 (4d6 +6) bludgeoning damage and be knocked prone . The Dragon can then fly up to half its flying speed.