Eldrantulku

Telepathic Bond. The fiend ignores the range restriction on its telepathy when communicating with a creature it has charmed. The two don't even need to be on the same plane of existence.

Shapechanger. The fiend can use a bonus action to polymorph into a Small, Medium, or Large humanoid, or back into its true form. Without wings, the fiend loses its flying speed. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Legendary Resistance (3/Day).  When Eldrantulku fails a saving throw they may choose to succeed at it instead. 

Magic Resistance. Eldrantulku has advantage on all saving throws against spells and other magical effects. 

Innate Spellcasting. Eldrantulku's Spellcasting ability is Charisma (Spell Save DC26, +18 to hit with spell attacks). She can innately cast the following spells, requiring no material components: 

At Will: charm person, counterspell, detect thoughts, enemies abound, fireball, shield,  thunderstep
3/Day Each: Disintegrate, Globe of Invulnerability,  Mass Suggestion, Mental Prison, Psychic Crush(UA), Scatter
1/Day Each: Gate, Mass Polymorph, Power Word Kill, Time Stop

Multiattack  Eldranktulku makes 4 melee attacks with their Claws.

Claws. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 6 (4d6 + 12) slashing damage.

Charm. One humanoid the fiend can see within 90 feet of it must succeed on a DC 24 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys the fiend's verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this fiend's Charm for the next 24 hours.

The fiend can have only one target charmed at a time. If it charms another, the effect on the previous target ends.

Draining Kiss. The fiend kisses a creature charmed by it or a willing creature. The target must make a DC 24 Constitution saving throw against this magic, taking 155 (20d12+50) psychic damage on a failed save, or half as much damage on a successful one. The target's hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.

Eldrantulku can take 3 legendary actions as detailed below. Yada yada you know the rest.

Attack. Eldrantulku can make two Claws attacks 

Draining Kiss (Cost 2 Actions). Eldrantulku can use it's Draining Kiss feature following the normal guidelines. 

Charm. Eldrantulku can use it's Charm ability following the normal guidelines.