Auril, The Frostmaiden

Legendary Resistance (6/Day). If the Frostmaiden fails a saving throw, she can choose to succeed instead.

Master of Magic. Auril has advantage on saving throws against spells and advantage on maintaining concentration. She can concentrate on up to 3 spells at once. She regains 1d10 spell slot or lower at the start of her turns.

Divine Being. Auril can't be surprised or be changed into another form against her will.  If she would be reduced to half hit points, she resets to 3 legendary resitances and regains all expended uses of Legendary Actions. Additionally, the she can now use the options in the "Mythic Actions" section for 1 hour. 

Fimbulvetr.  The Frostmaiden creates a magical blizzard in a 30-foot-radius sphere centered on herself. The area within the sphere is heavily obscured to those outside it, and the sphere moves with Auril. Any creature within this aura feels the freezing cold and cannot take reactions unless they succeed a DC 24 Constitution saving throw when they enter the Aura or begin their turn there. Additionally if they fail the saving throw they take 36 (8d8) cold damage, or half as much on a success. 

Frigid Aura. Whenever a creature within 5 feet of her hits the Frostmaiden with a melee attack, they take 2d8 cold damage. 

Champion of Ice. The Frostmaiden can move across icy surfaces without needing to make an ability check. She can see through areas obscured by snowfall, sleet, rain, and other wintry precipitation without penalty

Regeneration. The Frostmaiden regains 100 hit points at the start of her turn. If she has been hit by mistletoe, this trait doesn't function for 24 hours. The Frostmaiden dies only if she starts her turn with 0 hit points and doesn't regenerate.

Innate Spellcasting. Auril's innate spellcasting ability is Charisma (spell save DC 24). She is a 20th level spellcaster. She has a +15 to spell attack rolls. She can innately cast the following spells, requiring no material components:

At will: Cone of Cold, chill touch, ice knife, ray of frost, investiture of ice, wall of ice, ice storm. Rime's Binding Ice. She can use a higher level slot to upcast any of these spells innately. 

1st level (4 slots): silvery barbs, shield

2nd level (3 slots): hold person, mirror image

3rd level (3 slots): counterspell, dispel magic, spirit shroud

4th level (3 slots): banishment, blight

5th level (3 slots): destructive wave, scrying

6th level (3 slots): circle of death, globe of invulnerability

7th level (2 slots): finger of death, Tether Essence

8th level (1 slots): control weather, holy aura

9th level (1 slots): Blade of Disaster

Multiattack: The Frostmaiden can cast a spell and then use of the following actions.

Frozen Strike. Melee Spell Attack: +15 to hit, reach 5 ft., one creature. Hit: 13 (4d6) cold damage and 18 (4d6+5) slashing damage. The target must make a DC 24 Constitution saving throw, on a failure they are frozen (paralyzed). The target is frozen until it or another creature within reach of it uses an action to break away the ice. 

Cold Snap (Recharge 5-6). The Frostmaiden causes the temperature around her to drop dramatically. Each creature within 30 feet of the Frost Maiden must make a DC 24 Constitution saving throw. On a failure, a creature takes 78 (12d12) cold damage and is frozen (paralyzed) They are frozen until it or another creature within reach of it uses an action to break away the ice. On a success they take half as much damage and are not frozen. 

Fey Step. The Frostmaiden teleports, along with any equipment she is wearing or carrying, to an unoccupied space she can see within 30 feet of her. 

Attack. The Frostmaiden can use a a Blade of Disaster attack (if concentrating on that spell), or cast a bonus action spell like Blade of Disaster or Spirit Shroud. 

Fell Rebuke. In response to taking damage, Auril utters a fell word, dealing 10 (3d6) necrotic and 10 (3d6) cold damage to the attacker if they are within 30 feet of her, and Auril teleports, along with any equipment she is wearing or carrying, up to 30 feet to an unoccupied space she can see.

Cast a Spell. Counterspell, Shield, or Silvery Barbs.

The Frostmaiden has 6 legendary actions per round. 

Cantrip. Auril casts chill touch or ray of frost. 

Slaughter. Auril uses Fey Step or moves half her speed, then makes a Frozen Strike attack.

Divine Casting (Costs 2 Actions). Auril casts a leveled spell. Or regains a d10 spell slot or lower. 

Vengeance against Mortals (costs 3 actions). All creatures within 30 feet of the Frostmaiden is afflicted by entropic magic, taking 4d8 necrotic damage and healing the Frostmaiden by that much. Humans who took this damage also take 9 (2d8) necrotic damage at the start of each of its turns. Immediately after taking this damage on its turn, the target can make a DC 24 Constitution saving throw, ending the effect on itself on a success. Until it succeeds on this save, the afflicted humans can't regain hit points.

Rotted Fate (costs 2 actions). The Frostmaiden connects her spirit to a mortal within 60 feet of her. A crackling cord of negative energy tethers the Frostmaiden to the target. Whenever the Frostmaiden takes damage, the target must make a DC 24 Constitution saving throw. On a failed save, the Frostmaiden takes half the damage (rounded down), and the target takes the remaining damage. This tether lasts until the Frostmaidens next turn. 

Embrace of the Damned (Costs 3 Actions). The Frostmaiden causes necrotic magic to engulf one creature she can see within 60 feet of herself. The target must make a DC 24 Dexterity saving throw, on a failed save, the creature takes 36 (8d8) necrotic damage and 36 (8d8) cold damage and is restrained. They can repeat the saving throw on their turn to break free of their restraints.