Cold Absorbption. When the dragon is subjected to cold damage, it takes no damage and instead regains an amount of hit points equal to the cold damage.

Colossal. The dragon is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever the dragon makes a weapon attack, regardless of whether the attack hits or misses, each creature within 10 feet of the attack's target other than the target or the dragon itself must succeed on a DC 24 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The dragon's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Hunter's Prey. As a bonus action, the dragon can designate a creature within 300 feet of it as its prey. The dragon has advantage on all attack rolls against its prey and always knows the prey's exact location as long as it is on the same plane of existence. The dragon can have one prey at a time, and only when its current prey dies, it can designate a new one.

Ice Aura. The dragon emits an aura of bitter cold, which extinguishes all nonmagical flames within 20 feet of it and causes the ground within 20 feet of it to count as difficult terrain for creatures other than the dragon. At the start of the dragon's turn, all creatures within 20 feet of it take 14 (3d8) cold damage.

A creature also takes 14 (3d8) cold damage if it touches the dragon or hits it with a melee attack while within 20 feet of it.

Ice Walk. The dragon 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 it extra movement.

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

Spellcasting. The dragon is a 20th level spellcaster. It's spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). It can cast the following spells. requiring no material components:

• Cantrips (at will): ray of frost, shape water, mage hand, prestidigitation, thaumaturgy
• 1st level (4 slots): detect magic, identify, ensnaring strike, hunter's mark, shield
• 2nd level (3 slots): hold person, misty step, pass without trace
• 3rd level (3 slots), remove curse, sleet storm, slow
• 4th level (3 slots): death ward, elemental bane, locate creature
• 5th level (3 slots): summon draconic spirit, legend lore, scrying
• 6th level (2 slots): wall of ice, primordial ward
• 7th level (2 slots): forcecage, divine word
• 8th level (1 slot): control weather, power word stun
• 9th level (1 slot): time stop

Treasure Sense. The dragon knows the location of all magic items and of accumulations of mundane treasure within 5 miles of it.

Multiattack. The dragon can use its World-Ending Presence, and if it was flying at the start of its turn, it can use its Crushing Landing. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +16 to hit, reach 25 ft., one target. Hit: 22 (2d12 + 9) piercing damage plus 14 (3d8) cold damage and the dragon can grapple the target as part of that attack.

Claw. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) slashing damage plus 14 (3d8) cold damage. If the target is a creature, it must succeed on a DC 24 strength saving throw or be flung up to 3d4 × 10 feet away from the dragon, landing prone.

Tail. Melee Weapon Attack: +17 to hit, reach 35 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage plus 14 (3d8) cold damage and the target must make a DC 24 Strength saving throw or be knocked prone.

World-Ending Presence. Each creature of the dragon's choice that is within 300 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or drop whatever it was holding and become frightened for 1 minute. A frightened creature must use the dash action on each of its turns, moving as far away from the dragon as it can, using the safest possible route.  If a creature ends its turn in a loaction where it does not have line of sight to the dragon, it can repeat the saving throw, 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 the dragon's World-Ending Presence for the next 24 hours.

Crushing Landing. If the dragon is flying, it can attempt to land forcefully in a space of its choice, even if that space is occupied by other creatures. When it does so, its movement speed is reduced to 0 until the end of the turn, and each creature in the dragon's space or within 30 feet of it, other than the dragon itself, must make a DC 24 Dexterity saving throw or take 26 (4d12) bludgeoning damage plus 14 (3d8) cold damage and be knocked prone.

Cold Breath (Recharge 5–6). The dragon exhales cold in a 300-feet cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 90 (20d8) cold damage on a failed save, or half as much damage on a successful one. In addition, on a failed saving throw, a creature is encased in ice until the end of its next turn or until someone uses an action to break the ice off it. While encased in ice, a creature's movement speed is reduced to 0.

Quicken Spell (3/day). The dragon casts one of its spells of 5th level or lower with a casting time of 1 action.

Reflective Ice. If the dragon is targeted by a magic missile spell, a spell that requires a ranged attack roll or a line spell, it can use its reaction to reflect the spell back at the caster, as though it originated from the dragon, turning the caster into the target.

The dragon 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 dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 30 feet of the dragon must succeed on a DC 24 Dexterity saving throw or take 18 (2d8 + 9) bludgeoning damage plus 14 (3d8) cold damage and be knocked prone. The dragon can then fly up to half its flying speed.

Cast a Spell (Costs 2 Actions). The dragon casts one of its spells of 5th level or lower.

Blizzard (Costs 3 Actions). The dragon creates a blizzard around itself. Each creature within that area must succeed on a DC 24 Constitution saving throw, taking 27 (6d8) cold damage plus 14 (3d8) bludgeoning damage or half as much on a success. In addition, on a failed saving throw, a creature is knocked prone.