Bahamut, Grand Master of Flowers

Unarmored Defense. While the aspect is wearing no armor and wielding no shield, its AC includes his Wisdom modifier and proficiency bonus.

Evasion. When the aspect is subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, he instead takes no damage if he succeeds on the saving throw, and only half damage if he fails.

Platinum Brilliance (Recharges after a Short or Long Rest). If the aspect would be reduced to 0 hit points, his current hit point total instead resets to 500 hit points, he recharges all features that need to be recharged, and he regains any expended uses of Legendary Resistance. Additionally, the aspect can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the aspect of Bahamut after his Platinum Brilliance activates.

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

Unarmored Movement. Bahamut's speed increases by 30 feet while he is not wearing armor or wielding a shield.

He also has the ability to move along vertical surfaces and across liquids on his  turn without falling during the move.

Deflect Missiles. Bahamut can use his reaction to deflect or catch the missile when you are hit by a ranged weapon attack. When he does so, the damage he takes from the attack is reduced by 1d10 + 40.

If he reduces the damage to 0, he can catch the missile if it is small enough for him to hold in one hand and he have at least one hand free. If he catches a missile in this way, he can make a ranged attack with the weapon or piece of ammunition he just caught, as part of the same reaction. He makes this attack with proficiency, regardless of his weapon proficiencies, and the missile counts as a monk weapon for the attack, which has a normal range of 20 feet and a long range of 60 feet.

Boon of Immortality. Bahamut is immune to any effect that would age him, and he can’t die from old age.

Aspect of the Wyrm. Bahamut's allies within 10 ft. are resistant to radiant damage.

Multiattack. The aspect uses Frightful Presence and then makes three Forceful Strike attacks.

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 19 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.

Forceful Strike. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 11 (1d8 + 9) force, acid, cold, fire, or lightning damage plus 80 radiant damage, and if the target is a creature, it must succeed on a DC 27 Dexterity saving throw or be knocked prone and a DC 27 Constitution save or be stunned until the end of his next turn. This attack counts as magical.

Quickened Healing (recharge 4-6). The Master of Flowers rolls 1 of his hit dice and gains that amount plus his Constitution modifier hit points.

Spellcasting. The aspect's spellcasting ability is Wisdom (spell save DC 27, +19 to hit with spell attacks). The monk has the following spells prepared:

At will: light, sacred flame, thaumaturgy, spare the dying, cure wounds, mage hand, faerie fire, dancing lights, levitate

5/day: command, inflict wounds, shield of faith, daylight, moonbeam, summon draconic spirit, draconic transformation, fly

3/day: hold person, spiritual weapon, Fizban's platinum shield, darkness

1/day: Bigby's hand, astral projection, fireball, vitriolic sphere, gate

Breath Weapon (Recharge 6). The aspect uses one of the following breath weapons:

Exalting Breath. The aspect exhales the restoring winds of Mount Celestia in a 120-foot cone. Each creature in that area of the aspect’s choice regains 71 (13d10) hit points, and each creature in that area of the aspect’s choice that has been dead for no longer than 1 hour is restored to life with all its hit points.

Elemental Breath. The aspect exhales elemental energy in a 120-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 84 (13d12) damage or one of the following types: acid, cold, fire, or lightning on a failed save, or half as much damage on a successful one.

Platinum Breath. The aspect exhales radiant platinum flames in a 120-foot cone. Each creature in that area must make a DC 26 Dexterity saving throw, taking 66 (12d10) radiant damage on a failed save, or half as much damage on a successful one.

Patient Defense (recharge 3-6). Bahamut takes the dodge, disengage, hide, or dash action.

Empty Body (recharge 5-6). The monk aspect becomes invisible for one minute and is immune to all damage but force damage while invisible.

Healing Word (recharge 2-6). The Master of Flowers casts healing word at first level.

Aspect of the Wyrm. The monk's aura of draconic fury explodes outward, scorching all enemies within 10 ft. for 3d10 radiant damage.

Flurry of Blows (recharge 3-6). If the aspect hits on at least one Force Strike attack, it can make two more against a hit target for each hit.

Change Shape. The aspect magically transforms into any Medium or Smaller Beast or Dragon, while retaining his game statistics (other than his size). This transformation ends if the aspect is reduced to 0 hit points or if he chooses to end it on his turn.

Slow Fall.  Bahamut reduces any falling damage he take by 60.

The Master of Flowers 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 Master regains spent legendary actions at the start of his turn.

Attack. The polymorphed dragon makes one Forceful Strike attack.

Furious Spellcasting (costs 2 actions). The dragon uses Spellcasting. One target of the dragon's choice must succeed on a DC 27 Wisdom saving throw or become frightened of the wyrm until the end of the target’s next turn.