Magic Resistance. The amethyst dragon has advantage on saving throws against spells and other magical effects.

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

Innate Spellcasting (Psionics). The adult amethyst dragon's innate spellcasting ability is Intelligence. The adult amethyst dragon can innately cast the following spells, requiring no material components:

At will: invisibility, plane shift

Multi attack. The adult amethyst dragon uses its frightful prescience then makes a bite attack and two claw attacks. 

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 14 (2d8 + 6) piercing damage. 

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 9 (1d6 + 6) slashing damage. 

Tail. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 11 (1d10 + 6) 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 16 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.

Explosive gem (1/day). The adult amethyst dragon spits a crystalline lozenge  up to 75ft and it explodes in a 20ft radius where it lands. Each creature within the radius must make a DC 16 Dexterity saving throw or take 15 (5d6) bludgeoning damage. 

Concussion breath (Recharges 5/6). The adult amethyst dragon exhales a line of concussive force in a 5ft wide, 60ft long line. Each creature within the radius must make a DC 16 Dexterity saving throw or take 56 (14d8) force damage and knocked prone. Half as much damage on a successful save. 

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 (Costs 1 Action). The dragon makes a Wisdom (Perception) check.

Tail Attack (Costs 1 Action). The dragon makes a tail attack.

Teleport (Costs 2 Actions). The amethyst dragon teleports up to 100ft to a place that they can see that isn't currently occupied.