Adult Prismatic Dragon

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead. When Legendary Resistance is used against a spell, the Adult Prismatic Dragon can choose to reflect the effects of the spell back at the caster.

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: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage.

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

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 7) 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 17 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.

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

Prismatic Breath. The dragon exhales eight multicolored rays of light. Each ray is a different color and has a different power and purpose. Each creature in a 60-foot cone must make a DC 19 Dexterity saving throw. For each target, roll a d8 to determine which color ray affects it.

1. Red. The target takes 10d6 fire damage on a failed save, or half as much damage on a successful one.

2. Orange. The target takes 10d6 acid damage on a failed save, or half as much damage on a successful one.

3. Yellow. The target takes 10d6 lightning damage on a failed save, or half as much damage on a successful one.

4. Green. The target takes 10d6 poison damage on a failed save, or half as much damage on a successful one.

5. Blue. The target takes 10d6 cold damage on a failed save, or half as much damage on a successful one.

6. Indigo. On a failed save, the target is restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the spell ends. If it fails its save three times, it permanently turns to stone and is subjected to the petrified condition. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.

7. Violet. On a failed save, the target is blinded. It must then make a Wisdom saving throw at the start of your next turn. A successful save ends the blindness. If it fails that save, the creature is transported to another plane of existence of the GM's choosing and is no longer blinded. (Typically, a creature that is on a plane that isn't its home plane is banished home, while other creatures are usually cast into the Astral or Ethereal planes.) 

8. Special. The target is struck by two rays. Roll twice more, rerolling any 8.

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 (Perception) 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 20 Dexterity saving throw or take 14 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.