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

Unusual Nature. Yharon doesn't need to sleep, eat, drink, or breath. 

Non-Magical Immunity. Yharon is immune to non-magical damage.

Scorching aura. Any creature that starts its turn within 60 feet of Yharon must make a DC 27 CON save or take 4d8 fire damage.

Unyielding Flame. When Yharon deals fire damage with an attack the target takes Yharons strength modifier in fire damage at the start of their next turn. additionally, Yharon ignores fire resistance and treats immunity as resistance.

Rising Heat. at initiative count 20 all creatures in Yharons lair take 1d6 fire damage, this damage increases by 1d6 every round Yharon is in combat while in mythic mode.

Multiattack. Yharon can use its Frightful Presence. It then makes four attacks: one with its bite, one with its tail, and two with its claws.

Bite. Melee Weapon Attack: +20 to hit, 20-foot reach, hit; 3d12+10 piercing damage and 3d12 fire damage.

Claw. Melee Weapon Attack: +20 to hit, 20-foot reach, hit; 2d10+10 piercing damage and 2d10 fire damage.

Tail. Melee Weapon Attack: +20 to hit, 30-foot (40 in mythic) reach, hit; 2d10+10 bludgeoning damage and 1d10 fire damage. creatures hit by this attack are pushed 30 feet away.

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 27 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.

Scorched Earth (recharge 6). as an action Yharon exhales a 200-foot cone of fire all creatures in the cone must succeed on a DC 27 Dex saving throw or take 40d6 fire damage or half as much on a failed save. a creature reduced to 0 hit points by this is turned to ash.

as a bonus action Yharon can make a claw or tail attack.

Yharon can take 4 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.

Claw. Yharon makes a claw attack

Charge. Yharon charges its movement without provoking opportunity attacks leaving 1d4 motes of fire behind that then make a ranged spell attack (+20 to hit) against a creature within 60 feet dealing 4d10 fire damage on a hit.

Rising Slam (costs 2 actions). Yharon leaps up to its movement speed and slams into the ground, any creatures in its space when it lands must make a DC 27 dexterity saving throw on a failed save creatures take 5d10 bludgeoning and 1d10 fire damage and half as much on a successful save. creatures in this area are then pushed out of yharons space.

Wing Attack (Costs 2 Actions). Yharon beats their mighty wings, all creatures within 20 feet of Yharon must make a DC 27 dex save or take 2d12+10 bludgeoning and 2d12 fire damage and be knocked prone and pushed 20 feet away, taking half as much damage on a success and not suffering any other affects.

Burning Overflow (Costs 3 actions). Yharon designates 1/2 of the arena to burst with overflowing flame, the area fills with embers and detonates at the end of the next round. creatures in this area lose half their current health when it detonates.

when reduced to 0 hp Yharon regains all hitpoints and gains access to the following mythic actions for 1 hour:

Furious Claws (costs 2 actions). Yharon makes 3 claw attacks.

Blazing charge (costs 2 actions). Yharons charge instead creates 2d8 motes and he can change direction during the charge.

Inferno Slam (costs 3 actions). Yharon leaps up to its movement speed and slams into the ground, any creatures in its space when it lands must make a DC 27 dexterity saving throw on a failed save creatures take 5d10 bludgeoning and 1d10 fire damage and half as much on a successful save. creatures in this area are then pushed out of Yharons space. the next creature to attack Yharon succefully must succeed a DC 27 dex save against 6th level hellish rebuke.

Fire Blossom (Costs 4 actions).  Yharon designates 3/4 of the arena to burst with overflowing flame, the area fills with embers and detonates at the end of the next round. creatures in this area lose half their current health when it detonates. additionally Yharon summons a miniature sun above their head that makes a mote of fire attack against a creature Yharon can see at the beginning of Yharons turn. Yharon can only have one sun active at a time and previous ones vanish if a new one is made.