Balta, The Queen of Frost

Amphibious. The queen can breathe air and water.

Divine Act (5/day). If Balta fails an ability check or an attack roll, she can choose to succeed instead. 

Divine Resistance. If Balta fails a saving throw, she rolls a d12. If she rolls 10 or higher, the queen succeeds on the saving throw instead. Otherwise, she adds the rolled number to the saving throw.

Heat Absence (Recharges after a Short or Long Rest). If the queen would be reduced to 0 hit points, her current hit point total instead resets to 475 hit points, she recharges her Breath Weapons, and she regains any expended uses of Divine Act. Additionally, the queen can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 135,000 XP (270,000 XP total) for defeating Balta after her Heat Absence activates.

Hypothermia. Cold damage dealt by Balta has unique interactions with immunity, resistance and vulnerability to cold damage: creatures with cold damage immunity take half the damage instead of none; creatures with resistance take full damage instead of half; and creatures with vulnerability take triple damage instead of double.

Ice Walk. Balta 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.

Innate Spellcasting. Balta can innately cast the following spells, her spellcasting ability is Charisma (+18 to hit, spell save DC 26):
At will: Ray of Frost (17th level) as a bonus action, Detect Magic
3/day: Wall of Ice, Dispel Magic (6th level)
1/day: Time Stop

Three Heads. Balta can take up to three reactions per round, but only one per turn. She also has advantage on saving throws against being knocked unconscious and effects that deal psychic damage or otherwise affect her mind. 

Regeneration. Balta regains 30 hit points at the start of her turn.

Unusual Nature. The queen doesn't require food, sleep or drink; and she cannot be surprised.

Multiattack. The queen can use her Chilling Presence. She then makes one Bite attack and two Claw attacks.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., three targets. Hit: 21 (2d10 + 10) piercing damage plus 19 (3d12) cold damage.

Claw. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 21 (2d10 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. The queen can have only one creature grappled this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Chilling Presence. Each creature of the queen's choice that is within 120 feet of the queen and aware of her must succeed on a DC 26 Wisdom saving throw or take 9 (2d8) cold damage and become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success, or taking the same damage on a fail. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the queen's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). The queen uses one of the following breath weapons:

Glacial Breath. Balta exhales glacial frost in a 300-foot cone. Each creature in that area must make a DC 26 Dexteriy saving throw, taking 110 (17d12) cold damage on a failed save, or half as much damage on a successful one.

Stalling Breath. Balta exhales chilling gas in a 300-foot cone. For the next minute, the area is a difficult terrain and is heavily obscured. Each creature that are in the area, and any creatures that enter it for the first time on a turn or start their turn there until the effect ends, must make a DC 26 Wisdom saving throw, or take 10 (3d6) cold damage and be subjected to effects of the Slow for the duration or until it leaves the area. On a successful saving throw, the creature takes half the damage only. An affected creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success

Frostline (1/day). Balta exhales freezing air in a 600-foot-long, 15-foot-wide line. All creatures in the area are restrained, as they begin to turn into ice statues over the course of the next minute. A restrained creature must make a DC 26 Constitution saving throw at the end of each of its turns. If it successfully saves against this breath three times, the condition ends. If it fails its saves three times, it becomes an ice statue and is subjected to the petrified condition, which can be ended by any revival effects. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.

Change Shape. The queen magically transforms into any creature that is Gargantuan or smaller, while retaining her game statistics (other than her size). This transformation ends if Balta is reduced to 0 hit points or uses her action to end it.

Freezing Counterspell. Balta interrupts a creature she can see within 60 feet of herself that is casting a spell. If the spell is 5th level or lower, it fails and has no effect. If the spell is 6th level or higher, the queen makes a Charisma check (DC 10 plus the spell's level). On a successful check, the spell fails and has no effect. Whatever the spell's level, the caster takes 19 (3d12) cold damage if the spell fails.

Iceberg Revenge. Whenever a creature within 60 feet of Balta damages her, the queen can use a reaction to deal 22 (5d8) cold damage to the attacker. The attacker must succeed on a DC 26 Wisdom saving throw or become frightened of the queen until the end of her next turn. 

Balta 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. The queen regains spent legendary actions at the start of her turn.

Tail Attack. The queen makes a Tail attack.

Cold Spell (Costs 2 Actions). The queen innately casts Cone of Cold.

Wing Attack (Costs 2 Actions). Balta beats her wings. Each creature within 40 feet of the queen must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The queen can then fly up to half her flying speed or swim up to half her swimming speed. 

If the queen's Heat Absence trait has activated in the last hour, she can use the options below as legendary actions.

Absolute Zero (1/hour). Balta unleashes the ultimate frost. All creatures in a 60-foot-radius sphere centered on the queen must succeed on a DC 26 Constitution saving throw or take 58 (13d8) cold damage and be paralyzed by the cold for 1 minute. A paralyzed create can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success, or taking another 22 (5d8) cold damage on a fail.

Bite. The queen makes one Bite attack.

Icicle Swarm (Costs 2 actions). Chilling spears of ice plummet to the ground at four different points Balta can see within 900 feet of her. Each creature in a 20-foot-radius, 100-foot-high cylinder centered on each point must make a DC 26 Dexterity saving throw. A creature takes 49 (14d6) cold damage and 49 (14d6) piercing damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one icy shower is affected only once.

Living Iceberg (Costs 2 actions). The queen covers herself with flexible ice armour; her AC increases by 5 and she gains 30 temporary hit points until the start of her next turn. Additionally, the damage dealt by Balta's Iceberg Revenge reaction increases to 40 (9d8) for same duration. 

Frozen Ocean (Costs 3 actions). Balta freezes all water in a 60-foot-radius, 10-foot-high cylinder centered on a point she can see within 900 feet of her. Any creatures and objects previously in that water suffer 58 (9d12) cold damage and are restrained by the ice. Creatures, restrained by the ice, can use an action making a DC 26 Strength ability check in attempt to escape; other creatures can make the same action to try and pull a restrained creature or object out of the ice. Depending on how much of the creature or object is captured by the ice, it can require up to three successful rolls. The successes don't need to be consecutive or from the same source; keep track of them until the target collects enough.