Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Molten Core. Extreme heat radiates from the dragon causing any creature that deals damage to the dragon while with 5 feet of it to takes 11 (3d6) Fire damage. Additionally while with in lava the dragon regains 11 (2d10) Hit Points at the start of its turn.

Molten Earth. When the dragon uses its walking, climb or borrow Speed it leaves behind a 10-foot wide trail of molten earth behind it for the next minute before it cools off. The area is consider difficult terrain and any creature that enters the area for the first time each turn or starts its turn their takes 18 (5d6) Fire damage. The Molten earth cools down instantly when exposed to cold damage or water suches as rain.

Multiattack. The dragon makes two Molten Claws/Wing attacks. It then uses either it uses either its Tail Smash attack or its Ashen Wind action.

Molten Claws/Wing. Melee Attack Roll: +17, reach 20 ft. Hit: 22 (2d10 + 10) Slashing or Bludgeoning damage plus 11 (2d10) Fire damage and the target is knocked prone.

Tail Smash. Melee Attack Roll: +17, each creatures with in 5 foot-emanation center on a point with 30 feet of the dragon. Hit: 38 (5d10 + 10) Bludgeoning damage.

Ashen Wind. Constitution Saving Throw: DC 20, each creature in a 60-foot-long, 20-foot-wide Line. Failure: 11 (3d6) Poison damage and the creature is Poisoned until the end of their next turn. Failure or Success: 11(3d6) Fire damage and the area is heavily obscured until the start of the dragon's next turn

Lava Breath (Recharge 5–6). Dexterity Saving Throw: DC 20, each creature in a 60-foot Cone. Failure: 55 (10d10) Fire damage. Success: Half damage. Failure or Success: The ground with in the area becomes molten as stated in the Molten Earth trait of the dragon.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 18):

At Will: Detect Magic, Thunderwave (deals Fire damage), Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell)

Territorial. Trigger: the dragon deals damage to a another dragon or a creature of Gargantuan size. Responds: The damage ignores resistance and immunities of the target and it deals a extra 11 (2d10) Force damage.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Rend. The dragon makes one Molten Claws/Wing attack.

Exhaust. The dragon cast Thunderwave using Spellcasting.

Molten Shell (Recharge 6). The dragon AC is reduced by 5 but the damage dealt incessse by Molten core to (6d6) Fire damage and its it's non-flying speeds are doubled until the end of the dragons next turn.