Ancient Stellar Dragon

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

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 d10. On a 1 to 5, the spell has no effect. On a 6 to 9, the dragon is unaffected, and the effect is reflected back at the caster as though it originated from the dragon turning the caster into the target. On a 10, the dragon is unaffected, and the spell is reflected back at the caster and up to 8 other hostile creatures in range, with each new target suffering the full effect of the spell.

Magic Weapons. The dragon's weapon attacks are magical.

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

Innate Spellcasting. The dragon's spellcasting ability is Charisma (spell save DC 27). It can innately cast the following spells, requiring no material components:

At will: color spray, detect magic, dispel magic

3/day each: greater restoration, raise dead, scrying

1/day each: true resurrection, control weather, regenerate

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

Bite. Melee Weapon Attack: +20 to hit, reach 15 ft., one target. Hit: 24 (2d10 + 13) piercing damage. 

Claw. Melee Weapon Attack: +20 to hit, reach 10 ft., one target. Hit: 20 (2d6 + 13) slashing damage. 

Tail. Melee Weapon Attack: +20 to hit, reach 20 ft., one target. Hit: 22 (2d8 + 13) bludgeoning damage. 

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon 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 the dragon's Frightful Presence for the next 24 hours.

Prismatic Breath (Recharge 5-6). The dragon releases a blast of twelve multicolored rays in a 90 foot cone. Each creature in that cone must roll a d12 to determine which color ray effects them, and then roll a DC 27 saving throw.

1.White. Saving throw is Dexterity. The target takes 20d10 radiant damage on a failed save, or half as much on a successful one. A target which fails it's save is also blinded for 1 minute.

2.Red. Saving throw is Dexterity. The target takes 30d10 fire damage on a failed save, or half as much on a successful one.

3.Orange. Saving throw is Dexterity. The target takes 20d10 acid damage on a failed save, or half as much on a successful one. On a failed save, any non-artifact armor being worn by the target suffers a permanent, cumulative -1 penalty to AC. If the penalty reaches -5, the armor is destroyed. Any non-artifact items being wielded by the target suffers 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 20d10 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.

5.Green. Saving throw is Constitution. The target takes 20d10 poison damage on a failed save, or half as much on a successful one. A target which fails it's save is also poisoned until cured.

6.Turquoise. Saving throw is Constitution. The target takes 20d10 thunder damage on a failed save, or half as much on a successful one. On a failed save, the target's AC is reduced by 10 for 1 minute.

7.Blue. Saving throw is Constitution. The target takes 20d10 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 27 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, but retaining it's normal weight and having vulnerability to fire and bludgeoning damage. If it succeeds three times, the effect ends.

8.Indigo. Saving throw is Dexterity. The target takes 30d10 force damage on a failed save, or none on a successful one. A target reduced to 0 hitpoints by this attack is reduced to a pile of fine gray dust, along with any non-artifact items it was wearing or carrying.

9.Violet. Saving throw is Wisdom. On a failed save, the target is charmed by the dragon and under it's control for the next minute. It can repeat it's save at the end of each of it's turns, or any time it takes damage. On a successful save, or at the end of the minute, the effect ends and the target takes 10d10 psychic damage. 

10.Grey. Saving throw is Dexterity. The target takes 20d10 magical piercing damage on a failed save, or half as much on a successful one. On a failed save, the target gains vulnerability to bludgeoning, piercing, and slashing for the next minute. Immunities and resistances to those damage types are negated for 1 minute.

11.Black. Saving throw is Wisdom. The target takes 20d10 necrotic damage on a failed save, or half as much on a successful one. A target reduced to 0 hitpoints by this attack dies instantly, and cannot be resurrected by anything short of a wish spell.

12.Special. The target is struck by multiple rays. Roll twice more. If another 12 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 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 27 Dexterity saving throw or take 20 (2d6 + 13) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.