Celestial Dragon King Rumanor

Amphibious. Rumanor can breathe air and water.

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

Special Equipment. Rumanor wears the Robe of the Archmagi, a Ring of lightning resistance and Bracers of Defense.

Limited Magic Inmunity. Rumanor can't be affected or detected by spells of 3rd level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

Celestial Aura. At the start of each of Rumanor's turns, each creature within 5 feet of him takes 10 (3d6) radiant damage. A creature that touches Rumanor or hits him with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.

Spellcasting. Rumanor is an 20th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 24, +18 to hit with spell attacks). Rumanor can cast  the following sorcerer´s spells prepared:

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, chill touch

1st level (4 slots): detect magic, cure wounds, shield, absorb elements

2nd level (3 slots): mind spike, shatter, detect thoughts

3rd level (3 slots): counterspell, fireball, lightning bolt

4th level (3 slots): fire shield, blight, banishment

5th level (3 slots): cone of cold, immolation, dominate person

6th level (2 slot): mental prison, disintegrate

7th level (2 slot): fire storm, power word pain

8th level (1 slot): sunburst

9th level (1 slot): meteor swarm

Multiattack. Rumanor can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of Rumanor's choice that is within 120 feet of him and aware of it must succeed on a DC 24 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 Rumanor's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). Rumanor uses one of the following breath weapons.

Sunburst Breath. Rumanor exhales radiant fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 71 (13d10) radiant damage on a failed save, or half as much damage on a successful one.

Weakening Breath. Rumanor exhales gas in a 90-foot cone. Each creature in that area must succeed on a DC 24 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Change Shape. Rumanor magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (Rumanor's choice).

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

Detect. Rumanor makes a Wisdom (Perception) check.

Cantrip. Rumanor cast a cantrip.

Tail Attack. Rumanor makes a tail attack.

Power Wings (Costs 3 Actions). Rumanor´s wings begin to exhale magical energy. Each creature within 15 feet of Rumanor must make a DC 25 Dexterity saving throw  taking 30 (6d6 + 9) force damage or half as much damage on a successful one.