Ancient Sound Dragon

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

Resonant Rebirth (Recharges after a Short or Long Rest). If the sound dragon would be reduced to 0 hit points, its current hit point total instead resets to 350 hit points, and it recharges its Breath Weapon. Additionally, the sound dragon can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 62,000 XP (124,000 XP total) for defeating the sound dragon after its Resonant Rebirth activates.

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.

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 24 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.

Sonic Breath. The dragon exhales sound in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 71 (13d10) thunder damage on a failed save, or half as much damage on a successful one.

Infrasound Breath. The dragon exhales low frequency sound in a 90-foot cone. Each creature in that area must succeed on a DC 24 Constitution saving throw or the creature gains a point of exhaustion for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending all points of exhaustion caused by this 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.

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.

Tunnel Collapse (Costs 2 Actions). The sound dragon emits low frequency waves which causes a part of terrain to collapse. All players in a 15. foot radius must make a DC 22 Dexterity saving throw or take 10d10 bludgeoning damage and become trapped under the rubble. The player is knocked prone and it's speed is reduced to 0 until it makes a successful DC 22 Athletics check. 

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.