Ancient Storm Monarch

Ice Walk. The Storm Monarch 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. Furthermore, any exposed water within 30 ft. of the dragon instantly turns to ice, only thawing after the dragon is more than 30 ft. away from it.

Legendary Resistance (3/day). If the Storm Monarch fails a saving throw, it can chose to succeed instead.

Spellcasting. The Storm Monarch is a 17th level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). The Storm Monarch has following Sorcerer spells prepared:
Cantrips (at will): Ray of Frost, Shocking Grasp
1st level (4 slots): Fog Cloud, Sleep, Thunderwave
2nd level (3 slots): Gust of Wind, Shatter
3rd level (3 slots): Lightning Bolt, Sleet Storm
4th level (3 slots): Dimension Door, Ice Storm
5th level (2 slots): Cone of Cold, Hold Monster
6th level (1 slot): Chain Lightning
7th level (1 slot): Reverse Gravity
8th level (1 slot): Earthquake
9th level (1 slot): Time Stop

Multiattack. The Storm Monarch can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 22 (3d10 + 6) piercing damage plus 11 (2d10) lightning, cold or thunder damage (determined randomly).

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 16 (3d6 + 6) slashing damage plus 7 (2d6) lightning damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage plus 9 (2d8) thunder damage.

Frightful Presence. Each creature of the Storm Monarch'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 Storm Monarch's Frightful Presence for the next 24 hours. 

Breath Weapons (Recharge 5–6). The Storm Monarch uses one of the following breath weapons.

Cold Breath. The Storm Monarch exhales an icy blast in a 120-foot cone. Each creature in that area must make a DC 26 Constitution saving throw, taking 90 (20d8) cold damage on a failed save, or half as much damage on a successful one.

Lightning Breath. The Storm Monarch exhales lightning in a 150-­-foot line that is 15 feet wide. Each creature in that line must make a DC 26 Dexterity saving throw, taking 110 (20d10) lightning damage on a failed save, or half as much damage on a successful one.

Cacophony Breath. The Storm Monarch exhales sound in a 120-foot cone. Each creature in that area must make a DC 26 Constitution saving throw. On a failure, they take 35 (10d6) thunder damage and are stunned until the end of their next turn. On a success, they take half as much damage and are stunned only until the start of their next turn. In addition, each creature that failed the saving throw must make a DC 26 Wisdom saving throw at the start of their next turn. On a failure, they are incapacitated until they take any amount of damage.

The Storm Monarch 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 Storm Monarch regains spent legendary actions at the start of its turn.

Detect. The Storm Monarch makes a Wisdom (Perception) check with advantage.

Tail Attack. The Storm Monarch makes a tail attack, and also knocks the target prone on a hit.

Wing Attack (Costs 2 Actions). The Storm Monarch beats its wings. Each creature within 15 feet of the Storm Monarch must succeed on a DC 23 Dexterity saving throw or take 16 (3d6 + 6) bludgeoning damage and be knocked prone. The Storm Monarch can then fly up to its flying speed.