Adult purple dragon

Sunlight Sensitivity. While in sunlight, the purple dragon has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

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

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

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 14 (2d10 + 3) piercing damage plus 5 (2d4) psychic damage. 

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 10 (2d6 + 3) slashing damage. 

Tail. Melee Weapon Attack: +7 to hit, reach 15 ft., 1 target. Hit: 12 (2d8 + 3) 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 .

Psychic breath (recharges on 5 or 6). The purple dragon exhales a blast of psychic energy in a 60ft cone. Each creature in this area must make a DC 17 intelligence saving throw or take 42 (12d6) psychic damage and the target is stunned until the end of their next turn. Half as much damage on a successful save. 

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 adult purple 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 adult purple 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 ft. of the dragon must succeed on a DC 15 Dexterity saving throw or take 10 (2d6 + 3) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.