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.
Multi-Headed. The dragon has advantage on Wisdom(Perception) and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. The dragon has two additional reactions that can only be used to make opportunity attacks.
Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: two with its claws and one with its tail.
Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 14 (2d6 + 7) slashing damage.
Tail. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage.
Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 15 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Frightful Presence for the next 24 hours.
The dragon 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 creature's turn. The dragon regains spent legendary actions at the start of its turn. The dragon's legendary action options are associated with each of it's three heads (a bite and a breath weapon for each). Once the dragon chooses a legendary action option for one of its heads, it can't choose another one associated with that head until the start of its next turn.
White Head Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 9 (2d8) cold damage.
Blue Head Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 11 (2d10) lightning damage.
Black Head Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 9 (2d8) acid damage.
Cold Breath (Costs 2 Actions) The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 21 Constitution saving throw, taking 49 (11d8) cold damage on a failed save, or half as much damage on a successful one.
Acid Breath (Costs 2 Actions) The dragon exhales acid in a 90-foot line that is 10 feet wide. Each creature in that area must make a DC 20 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.
Lightning Breath (Costs 2 Actions) The dragon exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that area must make a DC 21 Dexterity saving throw, taking 60 (11d10) lightning damage on a failed save, or half as much damage on a successful one.
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