Claret Fog-Born Greatwyrm

Ashen Fliers. The dragon can breathe in any area that has an amount of air or smoke, regardless of how thin. It can hold its breath indefinitely.

Claret Scales. The dragon's flight speed is doubled while in smoke. In smoke, if the dragon moves at least 120 feet in a straight line before attacking, the attack deals an additional 46 (13d6) damage. The dragon cannot swim, but it can fly through water at half its fly speed.

Fade Into Fog. The dragon can use its action to shapechange into a mass of semisolid smoke (or back to its dragon form). While in this form, the dragon makes stealth checks at advantage and loses its walking and climbing speeds, but gains the hover property.

Spellcasting. The dragon is an innate spellcaster. Its spellcasting ability is charisma (spell save DC 26, +17 to hit with spell attacks). The dragon knows the following spells and can cast them at their lowest levels without material or somatic components:

Cantrips (at will): Fire Bolt, Produce Flame, Mage Hand, Druidcraft

7x/day: Burning Hands, Chromatic Orb (always chooses fire damage)

5x/day: Fireball, Melf's Minute Meteors

4x/day: Wall of Fire, Immolation, Prismatic Spray (always chooses fire damage)

2x/day: Illusory Dragon (always chooses fire damage) 

Multiattack. The dragon makes four attacks: one with its bite and three with its tail.

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 8) piercing damage plus 36 (8d8) fire damage.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft, one target. Hit. 41 (5d12 + 8) bludgeoning damage.

Smoke Breath (Recharge 5-6). The dragon exhales a suffocating wall of burning smoke in a 60 foot cone in front of it. Each creature in that area must make a DC 25 Constitution saving throw, taking 113 (25d8) fire damage on a failed save, or half as much damage on a successful one. The area of the cone is heavily obscured to all creatures but the dragon for the next round, and the dragon has three-quarters cover.

Dodge. The greatwyrm takes the dodge action as a bonus action. Once the dragon is hit with an attack, the dodge ends for the rest of the attacking creature's turn.

Burst. The dragon may use its reaction to burst in a cloud of smoke when a creature enters its reach. When the dragon uses its reaction in this way, it has three-quarters cover for the duration of the triggering creature's turn.

Flaming Form. When struck with a weapon or spell attack, the dragon unleashes a sphere of blazing fire from its form. All creatures of the dragon's choosing within fifteen feet must make a DC25 Constitution saving throw or take 72 (18d6) fire damage, half on success.