Multiattack

The dragon uses its Frightful Presence. It then makes one Bite attack and two Fin attacks.

Bite

Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 14 (4d6) cold damage.

Fin

Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.

Tail

Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) 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 20 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.

Tidal Breath (Recharge 5-6)

The dragon exhales a crushing wave of frigid seawater in a 90-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw. On a failure, the creature takes 44 (8d10) bludgeoning damage and 44 (8d10) cold damage and is pushed up to 45 feet away from the dragon and knocked prone. On a success, the creature takes half the damage and isn't pushed or knocked prone.