Amell Vetiver

Bloodsense. The vampire can sense living creatures that have blood or similar vital fluids in a radius of 240 feet.

Shapechanger. As a bonus action, the vampire can polymorph into a Medium cloud of mist, or back into its true form. While in mist form, the vampire can't speak or manipulate objects. It is weightless, has a flying speed of 60 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 thows, and it is immune to all nonmagical damage.

Enigmatic Nature. The vampire is immune to effects that turn undead, as well as any divination spell that it refuses.

Magical Attacks. The vampire's weapon attacks are magical.

Warlock Bond. The vampire is a warlock patron, and can cast sending at will as a free action if the target is its warlock, though the ability is innate, has no verbal, somatic, or material components, and does not count as a spell for the purposes of spells and features such as detect magic and antimagic field.

Spellcasting. The vampire is a 14th-level spellcaster. Its spellcasting ability is [Intelligence] (spell save DC #24, +14 to hit with spell attacks). He has following Wizard spells prepared:
Cantrips (at will): mending, message, prestidigitation, toll the dead, mage hand
1st level (# slots): [spell name], [spell name] 

Multiattack. The vampire makes three melee attacks, only one of which can be a Bite attack.

Claws (Vampire and Monstrous Form Only). Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 24 (3d10 + 8) [slashing] damage. Instead of dealing damage, the vampire can grapple the target [escape DC 21].

Bite. Melee Weapon Attack: +15 to hit, range 5 ft., one creature that is grappled by the vampire, incapacitated, or restrained. Hit: 21 (3d8 + 8) [piercing] damage and the target's Constitution score is reduced by 1d6. The target dies if this reduces its Constitution to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

Cast a Spell. The vampire casts a spell from its prepared Spellcasting list.

Blood Drain (Mist Form Only). One creature in the vampire's space must make a DC 21 Constitution saving throw (undead and constructs immediately succeed). On a failed save, the target takes 22 (4d6 + 8) necrotic damage, its hit points are reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. This reduction to the target's hit point maximum lasts until the target finishes a long rest. It dies if this effect reduces its hit point maximum to 0.

Blood Cauldron (Recharge 5-6). The vampire targets a living creature that has blood or similar vital fluids that it can see within 30 feet of it, making its blood boil. The target must make a DC 21 Constitution saving throw, taking 27 (5d10) necrotic damage plus 27 (5d10) fire damage on a failed save, or half as much damage on a successful one. If the saving throw fails by 5 or more, the target instead takes 50 necrotic damage plus 50 fire damage.

Special Trait . Enter the description for your special trait.

Cast a Cantrip. The vampire casts a cantrip it knows.

Cast a Spell. The vampire casts a spell from its Spellcasting list with a casting time of 1 Bonus Action. It cannot do so if it took the Cast a Spell action on this turn.

Legendary Reaction. The vampire has the following Legendary Reaction, though it shares its Legendary Action cost with Legendary Actions it possesses.

Shadowmeld (Costs 1 Action). Once per long rest while Octavia is within 60 feet, recharging at midnight, as a reaction to either the vampire or her taking damage, the vampire can call Octavia to join with him. The triggering damage is negated, as is any damage they would take while in this joined shadow form. They become incorporeal and invulnerable shadows, and Octavia rushes to his location and occupies the same space as him, moving with him for the duration. They become a Large swirling cloud of darkness that counts as magical darkness for any creature that enters the space. 
At the start of Octavia's turn, she can choose to immediately Invoke: Void Trespass for free on one enemy within 30 feet, regardless of her current Blood Points. Otherwise, she can choose to immediately reform behind one creature within 5 feet of the vampire, Surprising the target. She can also choose to reform in any space up to 30 feet away from him.
She can remain joined with the vampire for a turn, but at the start of her second turn while joined, she must separate, choosing one of the options.

Enter the description for how legendary actions work for your monster here.

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

Claws. The vampire makes one attack with its claws.

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

Arcane Reflex (Costs 2 Actions). The vampire casts a Cantrip or a Spell of 2nd level or below.

Blood Call (Costs 3 Actions). The vampire targets up to two living creatures that have blood or similar vital fluids that it can see within 30 feet of it. Each target must succeed on a DC 21 Constitution saving throw or take 18 (4d8) necrotic damage. A target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to the total damage dealt to all targets. The reduction lasts until the target finishes a long rest. A target dies if this effect reduces its hitpoint maximum to 0.