Innate Spellcasting. The dragon's spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: light, detect magic, guiding bolt

3/day: color spray, dispel magic, lesser restoration, mass healing word

1/day: heal, raise dead, crown of stars

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

Magic Resistance. The dragon has advantage on saving throws against spells and other magical effects.

Magic Weapons. The dragon's weapon attacks count as magical for the purposes of overcoming resistances and immunities.

Reflective Carapace. Any time the dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d8. On a 1 to 5, the spell has no effect. On a 6 to 8, the dragon is unaffected, and the spell is reflected back at the caster as though it originated from the dragon turning the caster into the target.

Multiattack. The dragon makes three attacks: one with it's bite and two with it's claws.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit:  14 (2d6 + 7) slashing damage plus 7 (2d6) radiant damage. 

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 7 (2d6) psychic damage, and the dragon can grapple the target as part of the attack.

Tail. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage plus 7 (2d6) force damage, and the target must succeed on a DC 20 Strength saving throw or be knocked prone.

Prismatic Breath (Recharge 5-6). The dragon releases a blast of ten multicolored rays in a 30 foot cone. Each creature in that cone must roll a d10 to determine which color ray effects them, and then roll a DC 19 saving throw.

1.White. Saving throw is Dexterity. The target takes 55 (10d10) radiant damage on a failed save, or half as much on a successful one. On a failed save, the target is also blinded for 1 minute. The target can repeat the saving throw at the end of each of it's turns, ending the effect on itself on a success.

2.Red. Saving throw is Dexterity. The target takes 88 (16d10) fire damage on a failed save, or half as much on a successful one.

3.Orange. Saving throw is Dexterity. The target takes 55 (10d10) acid damage on a failed save, or half as much on a successful one. On a failed save, any nonmagical armor being worn by the target suffers a permanent, cumulative -1 penalty to AC. If the penalty reaches -5, the armor is destroyed. Any nonmagical items being wielded by the target suffer a permanent, cumulative -1 penalty to attack and damage rolls. If the penalty reaches -5, the item is destroyed.

4.Yellow. Saving throw is Constitution. The target takes 55 (10d10) lightning damage on a failed save, or half as much on a successful one. A target which fails it's save is also paralyzed for 1 minute. The target can repeat the saving throw at the end of each of it's turns, ending the effect on itself on a success.

5.Green. Saving throw is Constitution. The target takes 55 (10d10) poison damage on a failed save, or half as much on a successful one. A target which fails it's save is also poisoned for 24 hours.

6.Blue. Saving throw is Constitution. The target takes 55 (10d10) cold damage on a failed save, or half as much on a successful one. On a failed save, the target is restrained. It must then make a DC 19 Constitution saving throw at the end of each of it's turns. After three failed saves, the target is permanently turned to ice, becoming petrified. While petrified in this way, the target retains it's normal weight and has vulnerability to fire and bludgeoning damage. If it succeeds three times, the effect ends. The effect also ends if the target takes 40 fire damage before being petrified.

7.Indigo. Saving throw is Dexterity. The target takes 65 (10d12) force damage on a failed save, or none on a successful one. A target reduced to 0 HP by this attack is reduced to a pile of fine gray dust, along with any nonmagical items it was wearing or carrying.

8.Violet. Saving throw is Wisdom. On a failed save, the target is charmed by the dragon for 1 minute. While charmed in this way, the dragon can communicate with the target telepathically, and the target will carry out the commands given by the dragon to the best of it's ability, provided the command is not obviously self-harmful. The target can repeat the saving throw at the end of each of it's turns, or any time it takes damage. On a successful save, the effect ends on the target.

9.Black. Saving throw is Wisdom. The target takes 55 (10d10) necrotic damage on a failed save, or half as much on a successful one. A target reduced to 0 HP by this attack is killed instantly, and can only be revived by means of a wish spell.

10.Special. The target is struck by multiple rays. Roll twice more. If another 10 is rolled, the target is struck by an additional ray.

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 it's turns.

Detect. The dragon makes a Wisdom (perception)

Tail Attack (Costs 2 Actions). The dragon makes a tail attack.

Spell (Costs 2 Actions). The dragon casts a spell from it's 3/day or At Will list.