Amphibious.

The dragon can breathe both air and water.

Legendary Resistance (3/Day).

If the dragon fails a saving throw, it can choose to succeed instead.

Actions

Multiattack.

The dragon makes one Bite attack and two Claw attacks.

Bite.

Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 9 (2d8) force damage.

Claw.

Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 11 (1d8 + 7) slashing damage.

Singularity Breath (Recharge 5–6).

The dragon creates a shining bead of gravitational force in its mouth, then releases the energy in a 90-foot cone. Each creature in that area must make a DC 20 Strength saving throw. On a failed save, the creature takes 45 (10d8) force damage, and its speed becomes 0 until the start of the dragon's next turn. On a successful save, the creature takes half as much damage, and its speed isn't reduced.

Spellcasting (Psionics).

The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

1/day each: blink, control water, dispel magic, protection from evil and good, sending

Bonus Actions

Change Shape.

The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

Psychic Step.

The dragon magically teleports to an unoccupied space it can see within 60 feet of it.

Legendary Actions

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.

Claw. The dragon makes one claw attack.
Psionics (Costs 2 Actions). The dragon uses Psychic Step or Spellcasting.
Explosive Crystal (Costs 3 Actions). The dragon spits an amethyst that that explodes at a point it can see within 60 feet of it. Each creature within a 20-foot-radius sphere centered on that point must succeed on a DC 20 Dexterity saving throw or take 13 (3d8) force damage and be knocked prone.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon can take one of the following lair actions; the dragon can't take the same lair action two rounds in a row:

Beguiling Whisper. The dragon telepathically whispers to one creature within range of the dragon's telepathy. The creature must succeed on a DC 15 Wisdom saving throw or be charmed by the dragon until initiative count 20 on the next round. A creature charmed in this way obeys to the best of its ability any command the dragon issues that isn't directly harmful to the creature.
Imprisoning Force. The dragon casts the forcecage spell, using its spell save DC and requiring no spell components. The spell ends early if the dragon uses this lair action again or dies.
Spatial Projection. The dragon chooses a space it can fit into within the lair. It exists in its own space and the chosen space simultaneously until initiative count 20 on the next round. Whenever it moves or takes an action, it chooses which version of itself is moving or acting. If an effect or attack can target both of the dragon's spaces at the same time, the dragon is affected only once.

Regional Effects

The region surrounding a legendary amethyst dragon's lair is altered by the dragon's magic, creating one or more of the following effects:

Background Check. Once per day, the dragon can cast the legend lore spell, requiring no spell components, naming any person, place, or object within 1 mile of the lair as the spell's subject.
Crystal Profusion. Amethyst crystals and geodes form along muddy shores and lake beds within 6 miles of the lair.
Thriving Wildlife. Fish and other aquatic Beasts reproduce rapidly and thrive in bodies of water within 6 miles of the lair. Foraging in these waters yields twice the usual amount of food.
Watery Sight. Water within 6 miles of the lair is a conduit for the dragon's psionic presence. As an action, the dragon can cast the clairvoyance spell, requiring no spell components and targeting any body of water in that region.

If the dragon dies, the populations of aquatic life near the lair return to normal levels over the course of 1d10 days. The existing abundance of amethysts remains, but new crystals and geodes form at a normal rate.

Adult Amethyst Dragon