Fimbulwinter's Breath. At the start of each of the Guardian's turns each creature within 10 feet of the guardian takes 2d10 (2d10) cold damage. 

Innate Spellcasting. The Guardian’s innate spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). She can innately cast the following spells, requiring no material components:

At will: wall of ice

3/day: ice storm, cone of cold

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

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

Multiattack. The Guardian has three attacks: Two with it's Ice Axe and One with it's Kick attack

Ice Axe. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 11 (1d10 + 5) slashing damage plus 3d8 cold damage.

Kick. Melee Weapon Attack: +12 to hit, range 10 ft.., one target. Hit: 21 (2d10+5) bludgeoning damage, and the target is knocked prone if it fails a DC 19 Strength saving throw.

Ice Bolt. Range Weapon Attack: +12 to hit, reach 150ft., one target. Hit: 27 (4d10 + 5) cold damage.

Auril's Guardian 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. Auril's Guardian regains spent legendary actions at the start of her turn.

Ice Bolt. The Guardian makes an Ice Bolt Attack.

Kick (Costs 2 Actions). The Guardian takes a Kick Attack

Wall of Ice (Costs 3 Actions). The Guardian casts wall of ice.