Amphibious. The dragon can breathe air and water.
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: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage.
Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 17 (3d6 + 7) slashing damage.
Tail. Melee Weapon Attack: +12 to hit, reach 15 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 18 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.
Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.
Scalding Mist. The dragon exhales fiery blast of lingering mist in a 60-foot cone. The mist lightly obscures the area and remains for one minute, or until dispersed with a moderate or stronger wind (at least 10 miles per hour). Each creature in that area or begins their turn there must make a DC 20 Constitution saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant resistance to this damage.
Obscuring Mist. The dragon exhales a murky, opaque mist in a 60-foot cube. This mist heavily obscures the area and remains for 1 minute, or until dispersed wiith a moderate or stronger wind (at least 10 miles per hour).
Mist Form. The dragon magically polymorphs into a Huge cloud of mist, or back into its true form. It reverts to its true form if it dies.0
While in mist form, the dragon can't take any actions, speak, or manipulate objects. It is weightless, has a flying speed of 20 feet, can hover, and can enter a hostile creature's space and stop there. It has advantage on Strength, Constitution, and Dexterity saving throws and it is immune to all nonmagical damage.
Spellcasting (innate). The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 16):
1/day each: fog cloud, wind wall, control water, control weather
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.
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 10 feet of the dragon must succeed on a DC 20 Dexterity saving throw or take 14 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.