Colossal. The dragon is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever the dragon makes a weapon attack, regardless of whether the attack hits or misses, each creature within 10 feet of the attack's target other than the target or the dragon itself must succeed on a DC 27 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The dragon's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Fire Aura. The dragon is sheathed in flames and lava. At the start of the dragon's turn, all creatures within 20 feet of it take 21 (6d6) fire damage.

A creature also takes 21 (6d6) fire damage if it touches the dragon or hits it with a melee attack while within 20 feet of it.

In addition, a nonmagical weapon that hits the dragon is destroyed by the fire and immense heat immediately after dealing damage.

Fire Absorbtion. When the dragon is subjected to fire damage, it takes no damage and instead regains an amount of hit points equal to the fire damage. 

Lava Strider. The dragon can move across and swim through lava without taking damage at its full speed. In addition, if the dragon starts its turn fully submerged in lava, all its attacks deal an additional 21 (6d6) fire damage until the end of its next turn.

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

Spellcasting. The dragon is a 20th level spellcaster. It's spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). It can cast the following spells. requiring no material components:

• Cantrips (at will): control flames, fire bolt, mage hand, prestidigitation, thaumaturgy
• 1st level (4 slots): detect magic, identify, fog cloud, command, shield
• 2nd level (3 slots): scorching ray, Maximilian's earthen grasp, misty step
• 3rd level (3 slots): erupting earth, Ashardalon's stride, remove curse
• 4th level (3 slots): death ward, elemental bane, wall of fire
• 5th level (3 slots): summon draconic spirit, legend lore, scrying
• 6th level (2 slots): disintegrate, primordial ward
• 7th level (2 slots): delayed blast fireball, divine word
• 8th level (1 slot): earthquake, incendiary cloud
• 9th level (1 slot): meteor swarm

Treasure Sense. The dragon knows the location of all magic items and of accumulations of mundane treasure within 5 miles of it.

Multiattack. The dragon can use its World-Ending Presence, and if it was flying at the start of its turn, it can use its Crushing Landing. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 21 (6d6) fire damage and the dragon can grapple the target as part of that attack.

Claw. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) slashing damage plus 21 (6d6) fire damage. If the target is a creature, it must succeed on a DC 27 strength saving throw or be flung up to 3d4 × 10 feet away from the dragon, landing prone.

Tail. Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage plus 21 (6d6) fire damage and the target must make a DC 27 Strength saving throw or be knocked prone.

World-Ending Presence. Each creature of the dragon's choice that is within 300 feet of the dragon and aware of it must succeed on a DC 24 Wisdom saving throw or drop whatever it was holding and become frightened for 1 minute. A frightened creature must use the dash action on each of its turns, moving as far away from the dragon as it can, using the safest possible route.  If a creature ends its turn in a location where it does not have line of sight to the dragon, it can repeat the saving throw, 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 World-Ending Presence for the next 24 hours.

Crushing Landing. If the dragon is flying, it can attempt to land forcefully in a space of its choice, even if that space is occupied by other creatures. When it does so, its movement speed is reduced to 0 until the end of the turn, and each creature in the dragon's space or within 30 feet of it, other than the dragon itself, must make a DC 27 Dexterity saving throw or take 26 (4d12) bludgeoning damage plus 21 (6d6) fire damage and be knocked prone.

Fire Breath (Recharge 5–6). The dragon exhales fire in a 300-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 119 (34d6) fire damage on a failed save, or half as much damage on a successful one. In addition, on a failure a creature gains a level of exhaustion due to the intense heat.

Quicken Spell (3/day). The dragon casts one of its spells of 5th level or lower with a casting time of 1 action.

Evaporate. When the dragon takes cold damage or is damaged by water created by a spell or natural effect, it can use its reaction to evaporate said water or any ice created by the cold-damaging effect. If it does so, it takes no damage instead, and the area within 60 feet of the dragon becomes heavily obscured by vapor until the start of its next turn.

The dragon can take 5 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 30 feet of the dragon must succeed on a DC 27 Dexterity saving throw or take 19 (2d8 + 10) bludgeoning damage plus 21 (6d6) fire damage and be knocked prone. The dragon can then fly up to half its flying speed.

Cast a Spell (Costs 2 Actions). The dragon casts one of its spells of 5th level or lower.

Heat Wave (Costs 3 Actions). The dragon emits scorching heat. All creatures within 300 feet of the dragon must make a DC 27 constitution saving throw, with disadvantage if they are in direct contact with an object made of metal. On a failure a creature takes 42 (12d6) fire damage and gains a level of exhaustion. On a success, a creature takes half as much damage without any other effect.