Ancient Cloud Dragon

Storm Reborn (Mythic Trait; Recharges after a Short or Long Rest). If the Ancient Cloud Dragon is reduced to 0 hit points, it does not fall die or fall unconscious. Its current hit point total instead resets to 600 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the dragon can now use the options in the “Mythic Actions” section for 1 hour.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Magic Resistance. The Ancient Cloud Dragon has advantage on saving throws against spells and other magical effects.

Cloud Form. As an action, the Ancient Cloud Dragon can polymorph into a natural cloud or revert to its true form. While in cloud form, the dragon is immune to all damage except for damage from wind-based spells or effects, and it can't make any physical attacks, cast spells, or take any other actions that affect the physical world, but it can move at its fly speed and pass through small holes or narrow openings, even mere cracks. The transformation lasts until the dragon uses an action to revert to its true form or until the dragon dies.

Magical Weapons. The Ancient Cloud Dragon's weapon attacks are magical.

Innate Spellcasting. The Ancient Cloud Dragon's innate spellcasting ability is Charisma (spell save DC 23). It can innately cast the following spells, requiring no material components:

At will: detect magic, fog cloud, gust of wind, lightning bolt

3/day each: control weather, chain lightning, storm sphere

1/day each: storm of vengeance, illusory dragon

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: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) lightning damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage plus 14 (4d6) lightning damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage plus 14 (4d6) lightning 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 23 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.

Lightning Breath. The dragon exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that area must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Thunderous Roar Breath. The dragon unleashes a devastating roar in a 60-foot radius. Each creature in that area must make a DC 23 Constitution saving throw, taking 77 (14d10) thunder damage on a failed save, or half as much damage on a successful one, and be deafened for 1 minute. A deafened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Storm Retaliation. In response to being damaged, the dragon unleashes a retaliatory burst of lightning at the attacker. The aggressor must make a DC 22 Dexterity saving throw. On a failed save, the creature takes 22 (4d10) lightning damage and is blinded until the end of its next turn. On a successful save, the creature takes half the damage and isn't blinded.

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 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Thunderclap (Costs 2 Actions). The dragon creates a burst of thunderous sound centered on itself. Each creature within 30 feet of the dragon (excluding the dragon itself) must succeed on a DC 23 Constitution saving throw or be stunned until the end of their next turn.

If the dragon's Storm Reborn trait has activated in the last hour, it can use the options below as legendary actions.

Cataclysmic Bolt. The dragon chooses up to three creatures it can see within 120 feet of itself. A bolt of lightning streaks from the dragon to each target. Each creature must make a DC 23 Dexterity saving throw, taking 45 (10d8) lightning damage on a failed save, or half as much damage on a successful one. 

Tempest Wing Gust (Costs 2 Actions). The dragon beats its wings with such force that it creates a localized tempest. Each creature within 60 feet of the dragon must succeed on a DC 23 Strength saving throw or be pushed up to 60 feet away from the dragon and knocked prone. This gust extinguishes unprotected flames in the area, disperses fog, and can topple structures or trees. The dragon can then fly up to half its flying speed without provoking opportunity attacks.