Auril, The Frostmaiden True Form

Avatar. An avatar's weapon attacks are treated as magic weapons. Avatars are unaffected by antimagic fields, and are able to use all of their powers normally as though the antimagic field doesn't exist.

Multiple Forms. Auril can use her action to transform into her Icedawn form or back into her Frostmaiden form (both are considered her true form). In her Icedawn form, Auril is unable to use her Innate Spellcasting trait.

Mark of the Frostmaiden. A creature that touches Auril or hits her with a melee attack while within 10 feet of her takes 14 (4d6) cold damage and after gains vulnerability to cold. At the start of each of its turns, the affected creature can make a DC 25 Constitution saving throw, ending the effect on itself and becoming immune to it for 24 hours on a success.

While affected, the creature's flesh is marked with the symbol of Auril and suffers disadvantage on saves against Auril's Command Creatures of the Cold trait.

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

Ice Walk. Auril can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost her extra movement.

Legendary Resistance (5/Day). If Auril fails a saving throw, she can choose to succeed instead.

Innate Spellcasting (Frostmaiden Form Only). Auril's innate spellcasting ability is Charisma (spell save DC 25, +17 to hit with spell attacks). Auril can innately cast the following spells, requiring no components:

At will:  heat metal, conjure elemental (air and ice elementals only), control weather (cold only), control winds, dispel magic, fire shield (cold shield only), finger of death (cold damage), Otiluke's freezing sphere, frost fingers, ice knife, ice storm, sleet storm, wall of ice, wind walk, wind wall

Aura of Frozen Death (Icedawn Form Only). Auril radiates an aura of deadly cold. At the start of each of Auril's turns, each creature within 40 feet of her must make a DC 25 Constitution saving throw. On a failed save, a creature takes 21 (6d6) cold damage and can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the target can take either an action or a bonus action on its turn, not both. On a successful save, the creature takes half as much damage and isn't slowed. A target reduced to 0 hit points by this damage or that fails three consecutive saves against it dies and is instantly transformed into solid ice.

Limited Magic Immunity (Icedawn Form Only). Auril can't be affected or detected by spells of 6th level or lower unless she wishes to be.

Ice Armor (Icedawn Form Only). Auril gains a +5 natural armor bonus to her AC.

Divine Aura. Each creature of Auril's choice that is within 240 feet of Auril and aware of her must succeed on a DC 21 Wisdom saving throw or become frightened for as long as it is aware of her presence. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Auril's Divine Aura for the next 24 hours.

Multiattack. Auril makes two frostbrand attacks and casts a spell or uses Bestow Mark.

Frostbrand. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage plus 28 (8d6) cold damage.

Bestow Mark. Auril bestows the Mark of the Frostmaiden on a creature she can see within 60 feet of her, dealing 42 (12d6) cold damage instead of the regular damage dealt by the trait.

Command Creatures of the Cold. One creature Auril can see within 60 feet of her must succeed on a DC 23 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys Auril'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 power for the next 24 hours. Auril can only use this power on creatures that are naturally immune to cold damage, or have taken the form of a creature that is naturally immune to cold damage, or are affected by Mark of the Frostmaiden.

Auril can take 5 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. Auril regains spent legendary actions at the start of her turn.

Melee Attack. Auril makes a frostbrand attack.

Cast a Spell (Costs 2 Actions). Auril casts one of her innate spells.

Bestow Mark. Auril uses Bestow Mark.