Vampire Archmage

Magic Resistance. The vampire archmage has advantage on saving throws against spells and other magical effects.

Shapechanger: If the vampire isn't in sunlight or running water, it can use its action to polymorph into a Tiny bat or a Medium cloud of mist, or back to its true form.

While in bat form, the vampire can't speak, its walking speed is  5 feet, and it has a flying speed of 30 feet. Its statistics other than its size and speed are unchanged. Anything it is wearing changes with it, but nothing it's carrying does. It reverts to its true form when it dies.

While in mist form, the vampire can't take any actions, speak, or manipulate objects. It is weightless, and has a flying speed of 20 feet, can hover, and can enter a hostile creatures 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 is immune to all nonmagical damage, except the damage it takes from sunlight.

Vampire Weaknesses: The vampire archmage has the the following flaws: 

Forbiddance: The vampire can't enter a residence without an invitation from the occupants.

Harmed by running water: The vampire 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 vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralysed until the stake is removed.

Sunlight hypersensitivity: The vampire takes 20 radiant damage while it starts its turn in sunlight. While in sunlight it has disadvantage on attack rolls and ability checks.

Misty Escape: When it drops to 0 hit points outside its resting place, the vampire 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, it can't revert to its vampire form, and it must reach its resting place withing 2 hours or be destroyed. Once in its resting place, it reverts to its vampire form. It is then paralysed until it regains at least 1 hit point. After spending 1 hour in its resting place, it regains 1 hit point.

Regeneration: The vampire 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 vampire takes radiant damage, or damage from holy water, this trait doesn't function at the start of the vampires next turn.

Spellcasting. The archmage is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, to hit with spell attacks). The archmage can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): detect magic, identify, mage armor*, magic missile

2nd level (3 slots): detect thoughts, mirror image, misty step

3rd level (3 slots): counterspell, fly, lightning bolt

4th level (3 slots): banishment, fire shield

5th level (3 slots): cone of cold, scrying, wall of force

6th level (1 slot): globe of invulnerability

7th level (1 slot): teleport

8th level (1 slot): mind blank*

9th level (1 slot): time stop, gate

*The archmage casts these spells on itself before combat.

Multiattack(Vampire Form only): The Vampire makes two attacks, only one of which can be a bite attack.

Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 1d4 + 2 piercing damage.

Bite(Vampire or bat form only). Melee Weapon Attack: +9 to hit, reach 5 ft., one wiling creature, or a creature that is grappled by the vampire. Hit: 1d6 + 4 piercing damage plus 3d6 necrotic damage. The target's hit point maximum is reduced by how much necrotic damage was taken, and the vampire regains hit points equal to that same number. The reduction remains until the creature finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain this way  and then buried rises the next night as a vampire spawn under the vampires control.

Charm: The vampire targets one humanoid it can see within 30 feet of it. If the target can see the vampire, the target must succeed on a DC 17 Wisdom saving throw against this magic or be charmed by the vampire. The charmed target regards the vampire as a trusted friend to be heeded and protected. Although the target isn't under the vampire's control, it takes the vampire's requests or actions in the most favourable way it can, and is a willing target for the vampires bite attack. Each time the vampire or the vampires 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 vampire is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

Children of the Night(1/Day): The vampire magically calls 2d4 swarms of bats or rats, provided that the sun isn't up. While outdoors, the vampire can call 3d6 wolves instead. The called creatures arrive in 1d4 rounds, acting as allies to the vampire and obeying its spoken commands. The beasts remain for 1 hour, until the vampire dies, or until the vampire dismisses them as a bonus action.

The vampire 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 vampire regains spent legendary actions at the start of its next turn. 

Move: The vampire moves up to its speed without provoking opportunity attacks.

Dagger: The vampire makes one dagger attack.

Bite(Costs 2 actions): The vampire makes one bite attack.