Magic Resistance. The dragon has advantage on saving throws from spells

Spellcasting. The celestial dragon is a 5th-level spellcaster. Its spellcasting  ability is wisdom (spell save DC 14, +6 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

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage.
Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) slashing damage.
Breath Weapon (Recharge 5–6). The dragon uses one of the following breath weapon.
Holy Breath. The dragon exhales lightning in a 60- foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 55 (10d10) radiant damage on a failed save, or half as much damage on a successful one.
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.