Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Volcanic Aura. A cloud heat, fire, ash and flame surround the volcano dragon within 40 feet. A creature must make a DC 23 Dexterity saving throw saving throw when it enters the aura or ends its turn there. A creature takes 35 (10d6) fire damage on a failed save, or half as much damage on a successful one. Additionally, on a failed save, a creature must make a DC 23 Constitution save or be poisoned whilst in the aura. 

Illumination. The dragon sheds bright light in a 30-foot radius and dim light in an additional 30 feet.

Water Susceptibility. For every 10 feet the dragon moves in water, or for every 5 gallons of water splashed on it, it takes 1 cold damage.

Innate Spellcasting. The dragon’s innate spellcasting ability is Charisma (spell save DC 23). It can innately cast the following spells:

3/day: ashardalon's stride, hellish rebuke, incendiary cloud

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: +14 to hit, reach 20 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) fire damage. If the target is a creature or a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 15 (3d10) fire damage at the start of each of its turns.

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 15 (2d6 + 8) slashing damage plus 7 (2d6) fire damage.

Tail. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 18 (2d6 + 11) slashing damage plus 7 (2d6) fire damage.

Mass Heat Metal (1/day). The dragon applies the effects of the heat metal spell to up to 10 targets it can see within 40 feet of it for up to 1 minute. The dragon must maintain concentration on this effect, as though it were concentrating on a spell. 

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 23 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.

Magma Breath (Recharge 6). The dragon exhales magma in a 30-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one, as they are spattered in lava. A creature or object below the cone must also make a DC 23 Dexterity saving throw or take an extra 3 (1d6) bludgeoning damage for every 10 feet the lava fell, counting as having fallen from the cones highest point, to a maximum of 60 (20d6), as the lava falls on them. Surfaces in the area of the cone or surfaces the lava falls onto, are covered in lava. Creatures that start their turn in or move through the lava take 50 (10d10) fire damage as they wade through the lava.

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 10 feet of the dragon must succeed on a DC 24 Dexterity saving throw or take 15 (3d10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Extend Aura. The dragon extends its Volcanic Aura's range from 40 feet to 80 feet until the end of its next turn.