Ancient Celestial Dragon

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Innate Spellcasting. A celestial dragon's innate spell spellcasting ability is wisdom (spell save DC 22). The dragon can innately cast the following spells, requiring no material components:

At will: major image (6th-level version), wind walk, detect evil and good
3/day each: blade barrier, dispel evil and good, resurrection
1/day: commune, plane shift
Divine Awareness. The dragon knows if it hears a lie.
Magic Resistance. The dragon has advantage on saving throws against spells and other magic effects.
Spellcasting. The celestial dragon is a 20th-level spellcaster. Its spellcasting  ability is wisdom (spell save DC 22, +14 to hit with spell attacks). It has the following cleric spells prepared:

Cantrips (at will): light, mending, sacred flame, spare the dying, thaumaturgy
1st level (4 slots): command, cure wounds, sanctuary, protection from evil and good, bless
2nd level (3 slots): calm emotions, lesser restoration, silence
3rd level (3 slots): dispel magic, remove curse, sending
4th level (3 slots): banishment, death ward, divination
5th level (3 slots): greater restoration, mass cure wounds, scrying
6th level (2 slots): word of recall, heal
7th level (2 slots): divine word, fire storm
8th level (1 slot): control weather, holy aura
9th level (1 slot): true resurrection

Multiattack. The dragon can use its Inspiring Presence. It then makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 11 (2d10) radiant damage.
Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.
Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.
Holy Breath (Recharge 5–6). The dragon exhales a radiant beam in a 120-­‐foot line that is 10 feet wide. Each creature in that line must make a DC 24 Dexterity saving throw, taking 88 (16d10) radiant damage on a failed save, or half as much damage on a successful one.
Inspiring Presence (5/day). Each creature of the dragon's choice that is within 120 feet of the dragon and can see and hear it of it gain a d12 inspiration die that they can use on a d20 roll of their choice. 
Change Shape. The dragon 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 (the dragon's choice). 
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

The dragon 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 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 15 feet of the dragon must succeed on a DC 24 Dexterity saving throw or take 16 (2d6 + 9) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Blinding Radiance (Costs 3 Actions). The dragon spreads its wings and stands up tall and it's plumage glows bright as a sun. Brilliant sunlight flashes in a 60-foot radius out from the dragon. Each creature in that light must make a DC 22 Constitution saving throw. On a failed save, a creature takes 6d6 radiant damage and is blinded  for 1 minute. On a successful save, it takes half as much damage and isn't blinded . Undead have disadvantage on this saving throw. A creature that is blinded makes another Constitution saving throw at the end of each of its turns. On a successful save, it is no longer blinded . This spell dispels any darkness in its area that was created by a spell.