Amphibious.

The dragon can breathe both air and water.

Fabricate (1/Day).

The dragon can cast fabricate, requiring no spell components and using Intelligence as the spellcasting ability.

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 15 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 10 (3d6) necrotic damage.

Claw.

Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Desiccating Breath (Recharge 5–6).

The dragon exhales yellowish necrotic energy in a 90-foot cone. Each creature in that area must make a DC 20 Constitution saving throw. On a failed save, the creature takes 49 (14d6) necrotic damage and is weakened until the end of its next turn. A weakened creature has disadvantage on Strength-based ability checks and Strength saving throws, and the creature's weapon attacks that rely on Strength deal half damage. On a successful save, the creature takes half as much damage and isn't weakened.

Spellcasting (Psionics).

The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 19):

1/day each: antilife shell, bane, control water, create or destroy water

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.
Essential Reduction (Costs 3 Actions). The dragon targets a creature or an object not being worn or carried that it can see within 60 feet of it. The target must succeed on a DC 19 Constitution saving throw or take 40 (9d8) necrotic damage. If this damage reduces the target to 0 hit points, it crumbles to dust.

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.
Cancellation. The dragon chooses an active spell of 5th level or lower that it's aware of in the lair and ends the spell.
Negative Energy Infusion. Up to two creatures the dragon can see within the lair must each succeed on a DC 15 Constitution saving throw or take 14 (4d6) necrotic damage. Negative energy then infuses the lair until initiative count 20 on the next round. While the infusion lasts, creatures in the lair other than the dragon can't regain hit points.

Regional Effects

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

Crystal Profusion. Natural stone within 6 miles of the lair grows plentiful crystal formations and veins of topaz gemstones, particularly underground.
Negative Energy. When a creature finishes a long rest within 6 miles of the lair, the creature must first succeed on a DC 15 Constitution saving throw or be unable to reduce its levels of exhaustion or regain spent Hit Dice. Creatures resistant or immune to necrotic damage are immune to this regional effect.
Thriving Wildlife. Giant squid are attracted to the sea within 6 miles of the lair, migrating and hunting there in large numbers.
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 population of giant squid in the region returns to normal levels over the course of 1d10 days. The existing abundance of crystals and topazes remains, but new ones form at a normal rate.

Ancient Topaz Dragon