Ancient Prismatic Dragon

Innate Spellcasting. The dragon’s spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). The dragon can innately cast the following spells, requiring no material components:   At will: color spray  3/day: hypnotic pattern   1/day: prismatic wall.

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

Prismatic Immunity. At the start of each of turn, the dragon rolls 1d6 to determine its damage immunity for the round: 1 = fire, 2 = acid, 3 = lightning, 4 = poison, 5 = cold, 6 = thunder.'

Epic Monster: All attacks and abilities go through resistances and immunities.

Multiattack. The dragon makes three attacks: one with its bite, and two with its claws. 

Elemental Claw. +19 to hit, reach 20 ft., one creature. H i t : 32 (4d10+10) piercing damage plus 14 (4d6) damage of a random type. Roll 1d6: 1 (red) = fire, 2 (orange) = acid, 3 (yellow) = lightning, 4 (green)= poison, 5 (blue) = cold, or 6 (purple) = thunder damage.

Elemental Bite: +19 to hit, reach 20 ft., one creature. H i t: 32 (4d8+10) piercing damage plus 14 (4d6) damage of a random type. Roll 1d6: 1 (red) = fire, 2 (orange) = acid, 3 (yellow) = lightning, 4 (green)= poison, 5 (blue) = cold, or 6 (purple) = thunder damage.

Change Shape. The dragon magically polymorphs into a beast or humanoid that has a challenge rating no higher than its own, or back to 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.

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 be 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 Recharges every 2 rounds. The dragon exhales a prismatic spray in a 120-foot cone. Each creature in the area is subjected to the effects of a DC 26 Prismatic Spray suffering 157 (35d8) damage of the appropriate type when damaged by the spell’s effect.

Speed of light: Dodges any attack even if it can't miss.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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 (2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 27 Strength saving throw or take 24 (4d6+10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Spectrum (3 legendary actons and no longer able to take legendary actions) This dragon channels every element and emotion at once becoming the Spectrum of emotions. In this form, all attacks deal fire, acid, lightning, poison, cold, and thunder damage.