Azarn, True White Dragon

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. When on ice speed is increased to 80ft for all movement types.

Legendary Resistance (3/Day). Can choose to take no damage. After using this Azarn takes half damage for the rest of the battle but fails all check.

Quick Recovery. Instantly recovers from all status effects. (Takes up all actions and instantly ends Azarn's turn, cannot be used if you've already done anything this turn)

Relentless Strikes. Attacks with bite and claw until a failed attack.

Bite. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) cold damage.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Freezing Flame (Recharge 6). The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 180 (20d10) cold and fire damage on a failed save, or half as much damage on a successful one.

Bonus actions can only be used while Power of the tundra is active. Gets 1 bonus action per person (Max 3)

Rampage of White. (Azarn can only use Rampage of White if he used Rampage of the Tundra as a reaction) The dragon exhales and releases a massive aura of flames and ice in a 700 foot circle. Each creature in that area must make a DC 22 Constitution saving throw and a DC 22 Dexterity Saving throw, taking 300 (20d20) cold and fire damage on a failed save, or half as much damage on a successful roll.

 Can use 1 reaction per person (Max 3)

Match Wits. When someone tries to cast a spell on Azarn he can use match wits to counter the spell unless the caster succeeds on a DC 18 Wisdom saving throw.

Snarl. When someone tries to shoot him with any ranged weapon he can snarl making them fire with a disadvantage.

Rampage of the Tundra. If Power of the Tundra is active and someone lands a successful attack that deals 50 or more damage on his next turn he can use "Rampage of White" 

Azarn can use 2 legendary actions per turn, unless he has used Power of the Tundra in which case he can make 4. 

Faint Snow. everything with a resistance to cold heals 10d6 hit points and has advantage on its next role.

Calming Ice. All those who look upon feel calm and lay down there weapons, provides opportunity attack for Azarn.

Mocking Laugh. A terrifying last echoes through the air and everyone who isn't Azarn takes 5d6 damage.

Power of the Tundra (Can only be used once and lasts 15 turns). Changes the entire terrain to a tundra, Azarn has advantage on all rolls and everyone without cold resistance has disadvantage on all rolls, while in this tundra Azarn gains access to the following attacks ; 
           Arctic's cold grasp.  An iceberg forms around the target unless they suceed on a DC 22 Dexterity saving throw, if they roll bellow 10 they take 100 (10d12 + 20)  cold damage.
           Arctic's Punishment. If the target is captured in a iceberg then they take 50 flat damage and break free.