Aerix Vokototh, Herald of Winter

Mythic Resistance. Whenever Aerix fails a saving throw, he can choose to expend one of his unspent legendary actions to succeed instead.

100% Sorcery (1/Turn). Aerix is so adept at manipulating his magic that he can fundamentally change the structure of his magic. Whenever Aerix uses Spellcasting to cast a spell of 1st level or higher, he can choose to apply one of the following additional properties to that spell.
Auril's Touch. If damage dealt by this spell reduces its target to 0 hit points, that creature dies and rises as a coldlight walker under Aerix's control at the start of its next turn.
Eternal Rime. If the spell requires a saving throw, the target makes its initial saving throw against it with disadvantage. Additionally, if the target fails its saving throw by 10 or more, the duration of, or damage dealt by the spell is doubled.
Siphon Heat. The spell is empowered by Aerix's divine blessing causing it to deal cold damage rather than its normal damage. Additionally, Aerix regains hit points equal to half the cold damage dealt by the spell.

White Album. Aerix is constantly surrounded by extreme cold in a 30-foot radius centered on him. Against creatures in the affected area, Aerix cannot be surprised, attacks against him cannot be made with advantage, and he has an AC of 22.

???% (Recharges after a Short or Long Rest). If Aerix would be reduced to 0 hit points, he doesn't die or fall unconscious. Instead, he regains 350 hit points, he recharges his Vehement Blizzard, and he regains any expended Legendary Actions - and for the next hour, Aerix's Strength increases to 46 (+18), his Dexterity increases to 52 (+21), his Constitution increases to 54 (+22), he regains 25 hit points at the start of his turn as long as he has at least 1 hit point, and he radiates bright light for 30 feet and dim light for another 30 feet.

Additionally, Aerix can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 75,000 XP (150,000 XP total) for defeating Aerix after his ???% activates.

Multiattack. Aerix uses Vehement Blizzard (if available), Spellcasting, or Snowgrave. He then makes two Iceshock attacks.

Iceshock. Melee or Ranged Spell Attack: +15 to hit, reach 5 ft. or range 120 ft., one target. Hit: 34 (6d8 + 7) cold damage. If the target is a living creature that isn't a deity or titan, then it can only regain hit points from magic of 6th level or higher until the end Aerix's next turn.

Spellcasting. Aerix casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (save DC 23):

Constant: telekinesis

At will: detect magic, mage hand, scrying, ray of frost (4d8)
3/day each: animate dead (as an action), blindness/deafness, cone of cold, contagion, immolation (cold damage), vacuum, volcanic explosion (cold damage)
1/day each: abi-dalzim's horrid wilting, fire storm (cold damage), flesh to stone (ice instead of stone), harm, night sky

Vehement Blizzard (Recharge 5-6). Until the end of Aerix's next turn, all attacks against him are made with disadvantage. If a creatures attack misses, it takes cold damage equal to the damage its attack would've dealt. If a creature successfully hits Aerix, the attack deals half damage and the attacker takes 9 (2d8) cold damage.

Snowgrave. Aerix uses one of the following two techniques (an area affected by Aerix's Snowgrave becomes difficult terrain until the start of his next turn):

Perfect Freeze. Aerix targets a point he can see within 120 feet of him. Each creature of his choice within 30 feet of that point must make a DC 23 Constitution saving throw taking 28 (8d6) cold damage and is frightened of Aerix until the end of his next turn on a failed saving throw, or half as much damage and not becoming frightened on a successful save. A creature that fails the save by 5 or more is instead restrained for 1 minute.

Glacial Tide. Aerix unleashes a wave of snowy ice and glacial water in a 60 foot cone or in a 120 foot line that is 10 feet wide. Each creature of his choice within that area must make a DC 23 Dexterity saving throw, taking 14 (4d6) slashing damage plus 14 (4d6) piercing damage on a failed save, or half as much damage on a successful one. A creature that fails the saving throw is incapacitated until the start of its next turn - and a creature that fails its saving throw by 5 or more is instead blinded and deafened for 1 minute.

Command Tribe. Aerix targets up to three nonhostile creatures he is aware of within 120 feet of him. Each chosen ally can then use their reaction to move up to half their speed and make one melee attack.

Frostmaiden's Perseveration (Recharge 5-6). As a reaction to a creature within 60 feet of Aerix making a Death saving throw, he can force that creature to automatically succeed or fail its Death saving throw (his choice).

Gently Weeps. When Aerix is affected by a spell or magical effect, he can use his reaction to attempt a Charisma (Deception) check versus the source's spell save DC. On a success, Aerix is unaffected and a creature of his choice within 60 feet of him is affected instead.

Aerix 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 creatures turn. Aerix regains spent legendary actions at the start of his turn.

Freeze. Aerix uses Spellcasting to cast an at-will spell, or he makes a Iceshock attack.

Will of the Frostmaiden (Costs 2 Actions). Aerix uses Snowgrave or Spellcasting.

Cryokinetic Might (Costs 3 Actions). Aerix recharges, or uses Vehement Blizzard.

If Aerix's ???% trait has activated in the last hour, he can use the options below as legendary actions.

Aurora Borealis. Aerix magically conjures an illusory duplicate of himself in an unoccupied space he can see within 120 feet of him. This duplicate can be used as the origin point for any of Aerix's actions (i.e. Spellcasting, Iceshock, etc.). This duplicate remains until it is used as the origin for two spells or abilities, or it takes damage.

Unrestrained Cryokinesis (Costs 2 Actions). Aerix empowers the next spell he casts with his Spellcasting action. The next time Aerix uses Spellcasting to cast a spell of 1st level or higher that deals damage, deals an additional 3 dice of damage and he adds his Charisma modifier (+7) to one of the damage rolls.