Amphibious. The dragon can breathe air and water.

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Poison Dust. The dragons exoskeleton constantly sheds creating a almost invisible cloud of Poison in a 10-foot emanation around the dragon. This cloud follows the dragon but leaves a trail behind it that last for 1 minute or until its is disperses by a strong wind. When a creature starts its turn in this cloud it is subjected to the following effect. Constitution Saving Throw: DC 21, Failure: the creature has the poisoned condition for the next minuted and at the end of its turn it can repeated its saving throw. While poisoned this way the creatures speed is also reduced to halve. Failure or Success: While in the cloud the creature has disadvantage on Wisdom ability checks.

Multiattack. The dragon makes three Serrate Claws/Wings attacks. It can replace one of its attacks with its Accursed Bite. It then uses either it uses either its Tail Spear attack or its Fear Gas action.

Serrate Claws/Wings. Melee Attack Roll: +14, reach 10 ft. Hit: 16 (2d8 + 7) Slashing damage plus 9 (2d8) Acid damage.

Accursed Bite. Melee Attack Roll: +11, reach 5 ft. Hit: 18 (4d6 + 4) Piercing damage plus 14 (4d6) Poison damage and the target becomes Cursed with Viduotoxin. When cursed in this way the target loses all its damage/condition residences and immunities and after each hour upon being cursed the target gain one point of Exhaustion up to a maximum of 5.

Tail Spear. Melee Attack Roll: +14, reach 15 ft. Hit: 18 (2d10 + 7) Piercing damage plus 6 (1d10) Poison damage and the creature is subjected to the following effect. Constitution Saving Throw: DC 21, Failure: The target has the Unconscious condition for 10 minutes. This effect ends for the target if it takes non-poison damage or a creature within 5 feet of it takes an action to wake it.

Fear Gas. Wisdom Saving Throw: DC 21, each creature in a 40-foot-long, 15-foot-wide Line. Failure: 18 (4d8) Psychic damage and the target is Frightened until the end of its next turn. While Frightened the target is also consider Charmed by the dragon. 

Malice Breath (Recharge 5–6). Constitution Saving Throw: DC 21, each creature in a 60-foot Cone. Failure: 52 (8d12) Poison damage and the creatures Constitution modifier is reduced by 1d4, down to a minimum of 3 until it finishes a Short Rest. Success: Half damage only.

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

At Will: Detect Magic, Invisibility, Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell)

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Poison Shock. One creatures with 60 feet of the dragon that is poisoned by it take 18 (4d8) Poison damage and they have disadvantage one the next saving throw it makes before the end of the dragons next turn.

Pounce. The dragon moves up to half its Speed, and it makes one Serrate Claws/Wings attack.

Defence Stand. The dragon gains halve cover and imposes disadvantage on melee attacks on it until the start of its next turn.