Actions

Multiattack. The dragon makes three attacks: one with its Bite and two with its Claws.
Bite. Melee Weapon Attack: 1d20+11 to hit, reach 10 ft., one target. Hit: 2d10+6 piercing damage plus 1d8 cold damage. If the target takes any cold damage, he is also Immobilized until the end of his next turn.
Claw. Melee Weapon Attack: 1d20+11 to hit, reach 5 ft., one target. Hit: 2d6+6 slashing damage plus 1d8 cold damage.
Tail. Melee Weapon Attack: 1d20+11 to hit, reach 15 ft., all targets in a 15-foot cone. Hit: 2d8+6 bludgeoning damage.

Bonus Action

Aurora Beam (Recharge 5–6). The dragon exhales a concentrated blast of cold in a 60-foot line that is 5 feet wide. Each creature in that area must make a DEX saving throw, taking [dragon's current hit points] in cold damage on a failed save, or half as much damage on a successful one. The target is stunned until the end of its next turn regardless if it makes or fails the saving throw.   Tactical Combat Alternate. The beam travels from one target to the next. For each target hit damage increases, the beam can not travel more than 10 feet to the next target and expires after the 6th target is hit. Use this Link to easily calculate damage based on hit points.

Reactions

Chill Rebuke (Recharge when first bloodied). As a reaction to an enemy’s melee attack that deals damage to the dragon, all creatures within 20 feet of the dragon must make a CON save or take 1d10+12 cold damage and become slowed (as the slow spell) until the end of his next turn or are not slowed and take half damage on a success.   Sudden Flurry. If the dragon is hit by an attack that does cold damage, he may move up to his speed towards the attacker, knocking enemies out of the way (DEX save or fall prone, on a success, the PCs get an opportunity attack), and then makes a Bite attack against the triggering attacker. On a critical hit, this attack also stuns the target until the end of his next turn.

Legendary Actions

The dragon can take 3 legendary actions, which replenish at the start of his turn. The actions can only be taken at the end of another creature's turn, only one at a time.

• Detect. The dragon makes a Wisdom (Perception) check.
• Dragon Punch. The dragon makes a Tail attack.
• Treacherous Ice. Magical ice covers the ground in a 20-foot radius centered on a point the dragon can see within 120 feet of itself. The ice, which is difficult terrain for all creatures except blizzard dragons, lasts for 10 minutes or until the dragon uses this legendary action again.