Adult Prismatic Dragon

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

Light Immunity. The dragon is immune to light-based spells and effects such as prismatic spray spell or another Prismatic's Breath Weapon.

Unusual Nature. The dragon doesn’t need to eat, drink, sleep, or breathe.

Innate Spellcasting. The dragon's spellcasting ability is Charisma (Spell save DC 18, +10 to hit with spell attacks). The Dragon has the following spells prepared:

at will: dancing lights, light, faerie fire, daylight

1/day: prismatic spray

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: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 15 (5d6) radiant damage.

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 27 (6d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 25 (4d8 + 8) 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 21 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 exhales prismatic energy in a 60-foot cone, which manifests as eight multicolored rays of light. Each creature in that area must make a DC 21 Dexterity saving throw. On a failed save, the creature takes 72 (12d12) damage of a type chosen by the dragon: acid, cold, fire, lightning, or poison. On a successful save, the creature takes half as much damage.

Change Shape. The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

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.

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.

Brilliant Breath (Costs 2 Actions). The dragon emits a beam of brilliant light flashing out in a 5-foot-wide, 30-foot-long line. Each creature in the line must make a DC 14 Constitution saving throw. On a failed save, a creature takes 16 (4d8) radiant damage and is blinded until the start of the dragon's next turn. On a successful save, it takes half as much damage and isn't blinded.