Auril, the frostmaiden

Cold-natured: When Auril casts a spell that deals cold damage, she can add her charisma modifier to the damage.

Spellcasting. Auril is a 20th-level spellcaster. Her spellcasting ability is Charisma (spell save DC 26, +18 to hit with spell attacks). Auril has the following spells prepared:
Cantrips (at will): Chill touch, Frostbite, Ray of frost, Toll the dead, shocking grasp, sapping sting.
1st level (4 slots):Absorb elements , Command, Cure wounds, Disguise self, Hellish rebuke (Cold damage), Magic missile, shield

2nd level(3slots): Hold person, Prayer of healing, shadow blade, spiritual weapon.

3rd level (3 slots): counterspell, coldball, fear, fly, haste.

4th level (3 slots): ice storm, blight, banishment, death ward, dimension door.

5th level (3 slots): Bigby's hand, commune, immolation (cold damage), mass cure wounds.

6th level (2 slots) Heal, wall of ice

7th level (2 slots) Divine word, Fire storm (ice damage).

8th level (1 slot)glibness, reality break

9th level (1 slot) gate

Divine Skill: Whenever Auril makes an attack roll, ability check or saving throw, she can add 1d12 to her result, potentially causing the outcome to change.

Divine knowledge: Auril knows the location of every Auril supporter within 50 miles of her, and can see through their eyes. 

Magic Weapons: Auril's weapon attacks are magical.

Limited Magic Immunity. Unless she wishes to be affected, Auril is immune to spells of 6th level or lower. She has advantage on saving throws against all other spells and magical effects.

Discorporation: When Auril drops to 0 hit points or is killed, her body is destroyed but her essence returns to winter's hall, where a new body is constructed which will be ready by the next winter solstice.

Auril makes one slam attack, then casts a spell. Alternatively, Auril makes two frost beam attacks

Slam. Melee Weapon Attack: Auril slams the ground, targeting a point within 30 feet of her. Each creature within 10 feet of that point must succeed on a DC 23 dexterity saving throw or take 27(5d10) bludgeoning damage, as well as 10 (3d6) cold damage. A creature takes half as much damage on a successful save.

Frost Beam. RangedWeapon Attack: +12 to hit, range: 1 mile, 1 target. Hit: 60(10d10+ 5) cold damage.

When Auril takes fire or radiant damage, she can use her reaction to become immune to the damage, and regain a number of hitpoints equal to half the damage dealt, rounded down.

Auril can take 5 legendary actions. She can only use a legendary action at the end of another creatures turn. She regains expended legendary actions at the start of her turn.

Slam (Costs 1 Action). Auril makes one slam attack.

Ice Flurry (Costs 2 actions). Auril creates a momentary blizzard in a 100 foot radius centred on her. Each creature in the radius must make a DC 23 dexterity saving throw. On a failed save a creature takes 25 (10d4) piercing damage. On a successful save, a creature takes half as much damage. 

Cast a spell (Costs 2 actions): Auril casts one of her innate spells.

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