Ancient Amethyst Dragon

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

Fiery Attacks. The dragon's attacks are magical and deal an additional 11 (3d6) fire damage (included in attack descriptions).

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

Crystal Regrowth. At the start of each of its turns, while the dragon is within its lair and out of sunlight, the dragon heals 30 hit points.

Spellcasting. The dragon is a spellcaster. Its spellcasting ability is Charisma (spell save DC 22, +12 to hit with spell attacks). The dragon always has following spells prepared: 
5/day: Comprehend Languages, Detect Magic, Sending
3/day: Greater Invisibility, Dispel Magic, Confusion, Polymorph
1/day: Prismatic Wall

Multiattack. The dragon can use its Beguiling Presence. It then makes three attacks: one with its bite, one with its claw, and one with its tail. Alternatively, the dragon can use its Shard Throw and/or Intoxicating Breath abilities.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 7) piercing damage plus 11 (3d6) fire damage. On a hit, the target is grappled by the dragon's mouth until the dragon makes another bite attack, or until the targeted uses an action to get themselves free, assuming they succeed a DC 19 Dexterity check.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 25 (5d6 + 7) slashing damage plus 11 (3d6) fire damage.

Tail. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 25 (5d6 + 7) bludgeoning damage plus 11 (3d6) fire damage. On a hit, the target is knocked prone.

Shard Throw (Recharge 5-6). The dragon swings its tail, letting go of hundreds of shards of crystal that flies through the air. Each creature within a 40-foot cone of the dragon must make a Dexterity saving throw, taking 38 (7d10) bludgeoning damage plus 11 (3d6) fire damage on a failed save, or half as much on a successful one.

Beguiling Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and not aware of it must succeed on a DC 24 Wisdom saving throw or become charmed by the dragon for 24 hours. While charmed in this way, creatures see the dragon as if it is their ruler. They will follow commands of the dragon as long as it doesn't harm the creature. The charmed affect If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Alluring Presence for the next 24 hours.

Intoxicating Breath (Recharge 5–6). The dragon exhales a thick purple mist in a 60-­foot cone. Each creature in that line must make a DC 21 Wisdom saving throw. A creature that fails their saving throw is slowed until the end of their next turn. 

A slowed target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.

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. The dragon makes a Wisdom (Perception) check.

Attack. The dragon makes a claw or tail attack.

Spell Cast (Costs 2 Actions). The dragon casts a spell.