Adult Purple Dragon

Legendary Resistance (6/day). If the dragon fails a saving throw, it can choose to succeed instead.

Polymorph. As an action, the dragon can polymorph itself into any creature that is of size medium or large that is its same CR or less, or back into its true form. If the dragon is polymorphed its stats change to match that creature's stats, but its hit points and modifiers do not change.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks.

Bite. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 33 (3d10 + 17) piercing damage and 22 (4d10) fire damage. 

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 28 (2d10 + 17) slashing damage. 

Tail. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 30 (3d8 + 17) bludgeoning damage and must make a DC 25 Strength saving throw or be knocked prone. 

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must make a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of it's 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 a number of hours equal to its Wisdom modifier (minimum of 1).

Plasma Spear. Ranged Weapon Attack: +13 to hit, range 5/120 ft., one target. Hit: 234 (36d12) fire damage and all creatures within 20 ft. of the target must make a DC 24 Constitution saving throw or be blinded. Creatures within 5 ft. of the target makes this save with disadvantage. This bypasses all fire immunities and resistances.

Plasma Breath (Recharge 3-6). The dragon breathes plasma flames in a 60-foot cone or a 15/120-foot line. Each creature within that area must make a DC 22 Dexterity saving throw, taking 234 (36d12) fire damage. After this all creatures must make a DC 24 Constitution saving throw if they are within 30 feet of the breathe weapon or be blinded, creatures inside the breathe weapon automatically fail the save. This bypasses all fire immunities and resistances.

The dragon can make 5 legendary actions per round, choosing from the options below. Only one legendary action option can be used at a time. The dragon regains all expended uses at the start of the round.

Detect. The dragon makes a Wisdom (Perception) check with advantage.

Tail Attack. The dragon makes a tail attack with advantage.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 20 feet of the dragon must make a DC 21 Dexterity saving throw or take 23 (2d8 + 14) bludgeoning damage and be knocked prone. The dragon can them fly up to half its flying speed.