Dual Type. Yharix is an elemental and an undead

Flyby: Yharix does not provoke opportunity attacks.

Magic Resistance: Yharix has advantage on saving throws against spells or other magical effects

Legendary Resistance (3/day): Whenever Yharix fails a saving throw, it may choose to succeed instead.

Anti-Life Inferno: Yharix may move through other creatures spaces with no additional cost to movement. The first time it does so on a turn, the creature takes 4 (1d8) damage of a type matching Yharixs current element. Additionally, when any creature makes a melee attack against the phoenix, they take 4 (1d8) damage of a type matching Yharixs current element.

Blaze of Glory (1/long rest): When Yharix is reduced to 0 hit points, it explodes. All creatures within a 15 foot radius of it must succeed on a DC 22 constitution saving throw or take 51 (10d10) damage of a type matching Yharix's element, halved on a save. It then immediately regains 300 hit points and gains access to any features marked Mythic Phase. 

Final Stand (1/long rest) (Mythic Phase): When Yharix is reduced to 0 hit points, it explodes again. All creatures within a 15 foot radius of it must succeed on a DC 22 constitution saving throw or take 51 (10d10) damage of a type matching Yharix's element, halved on a save. It then regains 200 hit points and immediately beings its final stand. If it is not killed in one round after final stand begins, it explodes for a final time, killing all creatures of its choice within 500 feet and disintegrating them into a fine ash.

Burning Aura (Mythic Phase): Yharix has an aura of scorching flame in a 10 foot radius surrounding it. Any creature that ends they turn in it must succeed on a DC 22 constitution saving throw or take 13 (3d8) damage of a type matching Yharix's element.

Unstable Elemental:  At the start of Yharix's turn it changes its elemental type randomly between radiant, necrotic, and fire. Yharix is resistant to damage of the other two types.

Spellcasting. Yharix is a 16th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +14 to hit with spell attacks). Yharix has following [class name] spells prepared:
4/day: Guiding Bolt, Scorching Ray, Faerie Fire
3/day: Heat Metal, Hold person, Moonbeam, Fire Shield
2/day: Moonbeam (at 6th lvl), Fireball, Wall of Fire
1/day: Disintegrate, Fire Storm, Incendiary Cloud

Multiattack. Yharix makes 3 Claw attacks or 2 Flaming Feather attacks. Alternatively, it may cast a spell, then make one claw or flaming feather attack.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 20 (3d8 + 7) damage of a type matching Yharix's element.

Flaming Feather. Ranged Weapon Attack: +14 to hit, range 120 ft., 1 target. Hit: 16 (2d8 + 7) damage of a type matching Yharix's element.

Flaming Beam (Recharge 5-6): Yharix fire a 10 ft. wide beam of fire in 60 ft. line originating from itself. All creatures in its path must succeed on a DC 21 dexterity saving throw or take 25 (8d6) damage of a type matching Yharix's element.

Sun Barrage (recharge 5-6) (Mythic Phase): Yharix creates up to 6 orbs or elemental energy and fires them at different targets within 120 feet. Each target and all creatures within a 10 foot radius of them must succeed on a DC 21 dexterity saving throw or take 29 (9d6) damage of a type matching Yharix's element.

Retribution. In response to being hit by a melee attack, Yharix may make a Claw attack against the attacking creature

Yharix 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. Yharix regains spent legendary actions at the start of its turn.

Flame Dash: Yharix dashes forwards 60 feet in a straight line.

Claw: Yharix makes a claw attack

Feather Barrage (Costs 2 actions): Yharix makes a Flaming Feather attack against up to 4 different creatures

Wing Blast (Costs 2 actions): Yharix flaps its wings and creates a 20 foot cone of scorching air emanating from itself. All creatures in the cone must succeed on a DC 22 Strength saving throw or take 13 (4d6) damage of a type matching Yharix's element and be pushed 15 feet backwards

Spellcasting (1/round): Yharix casts a spell it can cast from its spell list.

Yharix can take 3 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature’s turn. Yharix regains spent mythic actions at the start of its turn. Mythic actions replace Legendary Actions once the mythic phase is activated

Blink: Yharix teleports up to 60 feet to a space it can see. All creatures within 10 feet of the space it left must succeed on a DC 21 constitution saving throw or take 13 (4d6) damage of a type matching Yharix's current element, halved on a save.

Double Slash: Yharix makes 2 claw attacks against 1 target

Burning Skies (Costs 2 actions). Yharix creates 3 5 ft. wide 60 ft. long lines anywhere within 120 feet of itself, then teleports to the end of each line and dashes through them. All creatures within the lines must succeed on a DC 21 dexterity saving throw or take 29 (9d6) damage of a type matching Yharixs' current element, halved on a save.

Mechanics:

Maelstrom: At the end of Yharixs' first turn, it creates a 10 ft. radius, 60 ft. tall tornado of fire in the center. At the start of each of Yharixs' subsequent turns the tornado moves 60 feet in a straight line towards any creature marked with Flamedrawer. Any creatures who enter the tornado's space for the first time on their turn or end their turn in it must succeed on a DC 22 dexterity saving throw or take 25 (8d6) damage matching Yharix's current element.

When Yharix enters Mythic Phase, a second fire tornado appears at the center of the field and a second creature is designated flamedrawer.

Elemental Shielding: The boss begins the encounter shielded. While shielded, it is immune to all damage except for the damage type of its shield (Radiant, fire, or necrotic) Its shield can be broken by destroying the 3 elemental locks surrounding it. Once all 3 elemetnal locks are destroyed, the boss goes into damage phase

Elemental Locks: Each is a small object that cannot be physically interacted with and is shielded, making them invulnerable. The locks can be either purple (necrotic), yellow (radiant), or orange (fire), and there can be no more than 2 of any color. The only way to destroy these locks is by attacking them with a matching Elemental Charge (explained below).

Elemental Plates: there are 3 10x10 ritual circles on the floor, each glowing with radiant, necrotic, and fire elemental energy respectively. Stepping on them spawns 2 ads of a type matching the plate they stand on. (radiant spark, necrotic spark, or fire spark) Killing one causes it to drop an elemental charge matching the type of ad it was. 

Elemental Charge: An elemental charge is a simple weapon with the light and thrown property and a thrown range of 60 feet. It always hits its target when thrown and deals 19 (6d6) damage matching the element of the spark that dropped it (radiant, necrotic, or fire). If it explodes while being held, it deals the same damage to the creature that was holding it.

Flamedrawer: At the end of Yharix's first turn, it designates 1 creature with Flamedrawer. While this buff is active, any fire tornadoes on the field move 60 feet towards the creature in a straight line at the start of Yharix's turn.