Adult Marble Dragon

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

Minor Reflective Carapace. Any time the Sapphire Dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d10. On a 1 to 5, the Sapphire Dragon is affected. On a 6 to 9, the Sapphire Dragon is unaffected. On a 10, the Sapphire Dragon is unaffected, and the effect is reflected back at the caster as though it originated from the Sapphire Dragon, turning the caster into the target.

Crystalline Magic. All attacks are made by the dragon are considered magical.

Immutable Mind. The marble dragon's mind is impervious to magic. They are immune to charmed and frightened. The marble dragon is immune any memory altering affects and is immune to all divination spells to read its thoughts. 

Multiattack. The dragon can use its shining radiance. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8)  piercing damage plus 7 (2d6) fire damage.

Claw. Melee Weapon Attack: to hit, reach 5 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Shining Radiance. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 Wisdom saving throw or become blinded 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 Shining Radiance for the next 24 hours.

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

Divine Breath. The dragon exhales fire in a 120-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) psychic damage on a failed save, or half as much damage on a successful one.

Mind Factor. The dragon exhales a mighty roar in a 120-foot sphere. Each creature of there chose in that area must make a DC 22 intelligence saving throw, on a failed save the affected creatures have disadvantage on all wisdom and intelligence saves, they also take 14 (4d6) psychic damage when an affected creature attempts to make a wisdom or intelligence save or check.

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 check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.