Ancient Pyroclastic Dragon

Innate Spellcasting. The dragon can innately cast the following spells, requiring no material components: 3/day—produce flame, sound burst; 1/day—fire storm, incendiary cloud, power word stun, pyrotechnics, shatter, shout, wall of fire, wall of stone. The dragon's saving throw DC is 24, and its spell attack bonus is +17.

Multiattack: The dragon can use its Frightful Presence. It the 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) fire damage.

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

Tail: Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) 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 21 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.

Ash Breath (Recharge 5-6): The dragon exhales superheated ash and crushing force in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking  32 (9d6) fire damage, and 32 (9d6) thunder damage on a failed save, or half as much damage on a successful one.

Disintegration Beam (Recharge 5-6): The dragon may choose to use its Disintegration Beam in place of its Ash Breath. If it does, it exhales plasma in a 120-foot line that is 5 feet wide. Each creature in that area must make a DC 24 Dexterity saving throw, or die instantly and turn to ash.

Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each 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 ft. of the dragon must succeed on a DC 24 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.