Efficient Lungs. The dragon can hold its breath for up to 100 years.

Legendary Resistance (2/Day).

If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. 

The dragon can use its Frightful Presence or Sandstorm. It then makes three attacks: one with its bite and two with its claws.

Bite. 

Melee Weapon Attack: +9 to hit, reach 10 ft., one creature.

Hit: 21 (2d6+6) piercing damage plus 5 (1d10)
acid damage.

Claw. 

Melee Weapon Attack: +9 to hit, reach 5 ft., one creature.

Hit: 15 (2d6+6)
slashing damage.

Frightful Presence. (1/day)

Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 17 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.

Illusory Oasis.

The dragon can make a 150 foot cubic area centred on a point within 300 feet of itself  look, sound, and smell like some other sort of natural terrain. Manufactured structures, equipment, and creatures within the area aren’t changed in appearance. The tactile characteristics of the terrain are unchanged, so creatures entering the area are likely to see through the illusion. If the difference isn’t obvious by touch, a creature carefully examining the illusion can attempt a DC 15 Intelligence (Investigation) check to disbelieve it. A creature who discerns the illusion for what it is, sees it as a vague image superimposed on the terrain. The illusion lasts for 1 hour or until the dragon dismisses it as a bonus action. 

Sandstorm (Recharge 6)

Centred on a point the dragon can see within 120 ft., a violent vortex of sand erupts and fills a 60 foot radius making the area heavily obscured for d4 rounds. When the creature enters the area for the first time on a turn or starts its turn there, it takes 4 (1d8)
bludgeoning damage and must succeed on a DC 15 Dexterity Saving Throw or be knocked prone.

Acid Dust Breath (Recharge 5–6)

The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 55 (6d10)
acid damage on a failed save, or half as much damage on a successful one.

Enter the description for how legendary actions work for your monster here.

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.