Vampiric Dragon

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 each of its turns 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, also, it cannot regain hit points from any of its actions until the end of its next turn. The dragon only dies if it is at 0 hit points and does not regenerate.

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:

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

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.

Unusual Nature. The dragon doesn't need to breathe, nor does it require food, drink, or sleep, although it gains no benefit from a long rest unless it successfully inflicted necrotic damage on a living creature within the last 12 hours.

Multiattack. The dragon can make one Bite attack and two Claw attacks using the Attack action.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one creature. Hit: 11 (1d10 + 6) piercing damage plus 10 (3d6) necrotic damage. The dragon heals for an amount equal to the necrotic damage dealt, and the target's hit point maximum is reduced by the same amount. If the target was a humanoid and its hit point maximum is reduced to 0 by this attack, it dies and becomes a Vampire Spawn under the dragon's control when the dragon finishes a long rest, provided the corpse is not within direct sunlight during any point between its death and the dragon's slumber is concluded.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) slashing damage.

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 13 (2d6 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw, or be knocked prone.

Leeching Breath Weapon (Recharge 6). The dragon exhales a noxious odor that drains the life out of creatures caught in its fumes, releasing a breath weapon that goes out in a 60 foot cone. Each creature in that area, that is missing any of its hit points, must make a DC 19 Constitution saving throw, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful save, then the dragon regains 42 (12d6) hit points. Also on a failed save, a creature's hit point maximum is reduced by an amount equal to the necrotic damage inflicted. If the target was a humanoid and its hit point maximum is reduced to 0 by this breath weapon, it dies and becomes a Vampire Spawn under the dragon's control when the dragon finishes a long rest, provided the corpse is not within direct sunlight during any point between its death and the dragon's slumber is concluded.

Bat Swarm Breath Weapon (Recharge 5-6). The dragon exhales a swarm of bats from its mouth that shriek loudly and shred anything in their wake, before the bats disperse, releasing a breath weapon that goes out in a 60 foot cone. Each creature in that area must make a DC 19 Dexterity saving throw, taking 49 (14d6) force damage on a failed save, or half as much damage on a successful save. Objects in that area that aren't being worn or carried also take this damage. Creatures that took damage from this breath weapon are also Deafened until the start of the dragon's next turn.

Misty Teleport. The dragon teleports, along with anything it is wearing and carrying, to an unoccupied space it can see within 60 feet of it.

Change Shape. The dragon transforms into a Tiny Bat, or back into its true draconic undead self. While in the form of a bat, its statistics remain unchanged, except for its Size, however, it cannot perform any of the Actions in its statblock, except its Bonus Actions and Bite attack (though in the form of a bat, its Bite attack only deals 1 piercing damage, along with its usual necrotic damage).

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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.

Draconic Fury. The dragon makes 1 Claw or Tail attack.

Teleport. The dragon uses its Misty Teleport.

Vampiric Fury (Costs 2 Actions). The dragon makes 1 Bite attack.

Hypnotic Presence (Costs 2 Actions). The dragon scatters its scales in an area of 60 feet around itself, that latch on to nearby creatures and attempt to lure them to the dragon's side for a moment. Each creature of the dragon's choice within 60 feet of it must succeed on a DC 21 Wisdom saving throw, or become Charmed by the dragon for 1 minute. A Charmed creature can repeat the saving throw at the end of each its turns, ending the condition on a successful save. If a Charmed creature takes any damage or is ever more than 60 feet away from the dragon, the condition ends. If a creature succeeds on the saving throw, whether immediately or repeated, it is immune to the dragon's Hypnotic Presence for 24 hours.