Ancient amethyst dragon

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 ancient amethyst dragon's innate spellcasting ability is Intelligence (DC 24). The ancient amethyst dragon can innately cast the following spells, requiring no material components:

At will: enhance ability, invisibility, plane shift, suggestion 

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

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 20 (3d8 + 8) piercing damage. 

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target. Hit: 14 (2d6 + 8) slashing damage. 

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., 1 target. Hit: 11 (2d10 + 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 18 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 ancient 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 18 Dexterity saving throw or take 39 (13d6) bludgeoning damage. 

Concussion breath (Recharges 5/6). The ancient amethyst dragon exhales a line of concussive force in a 10ft wide, 80ft long line. Each creature within the radius must make a DC 18 Dexterity saving throw or take 88 (22d8) force damage and knocked prone. Half as much damage on a successful save.