Arianne the Deciever

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

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

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

Spellcasting. Arianne is an 18th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). Arianne can cast disguise self and invisibility at will and has the following wizard spells prepared:

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

1st level (4 slots): false life, ray of sickness, mage armor, magic missile

2nd level (3 slots): ray of enfeeblement, mirror image, misty step, blindness/deafness

3rd level (3 slots): counterspell, animate dead, lightning bolt, vampiric touch

4th level (3 slots): banishment, wall of fire, blight, dimension door, stoneskin

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

6th level (1 slot): disintegrate

7th level (1 slot): teleport

8th level (1 slot): clone

9th level (1 slot): power word kill

*Arianne casts stoneskin and mage armor spells on herself before combat.

Dagger of Wounding. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage. 

Hit points lost to this weapon's damage can be regained only through a short or long rest, rather than by regeneration, magic, or any other means.
Once per turn, when you hit a creature with an attack using this magic weapon, you can wound the target. At the start of each of the wounded creature's turns, it takes 1d4 necrotic damage for each time you've wounded it, and it can then make a DC 15 Constitution saving throw, ending the effect of all such wounds on itself on a success. Alternatively, the wounded creature, or a creature within 5 feet of it, can use an action to make a DC 15 Wisdom (Medicine) check, ending the effect of such wounds on it on a success.
(Requires Attunement)

Claw (Fiend Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: (2d8 + 3) slashing damage.

Charm. One humanoid the fiend can see within 30 feet of it must succeed on a DC 17 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys Arianne'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 Arianne's Charm for the next 24 hours.

Arianne 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 15 Constitution saving throw against this magic, taking 32 (5d10 + 5) 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.

Legendary Resistance (3/Day).  If Arianne fails a saving throw, she can choose to succeed instead.

Legendary Actions. Arianne 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. Arianne regains spent legendary actions at the start of her turn.

Move. Arianne can move up to its speed without provoking opportunity attacks.

Claw (Fiend Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: (2d8 + 3) slashing damage.

Draining Kiss (Costs 2 actions). The fiend kisses a creature charmed by it or a willing creature. The target must make a DC 15 Constitution saving throw against this magic, taking 32 (5d10 + 5) 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.