Adult Purple Dragon

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

Mind Palace. The purple dragon has advantage on saving throws against being frightened, and magic can't put it to sleep. It can't be targeted by any spell that attempts to control its actions (including those that don't have a charm effect).

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 5 ft., one target. Hit: 23 (3d10 + 7) piercing damage.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 7) slashing damage.

Tail. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 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.

Psychic Breath (Recharge 5-6). The dragon exhales a thick cloud of purple smoke in a 30-foot cone. Each creature in the area must succeed on a DC 19 Wisdom saving throw or take 67 (15d8) psychic damage, or half as much on a successful save.

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.

Psychic Fortitude (Costs 2 Actions). The dragon ends one condition that is affecting it.