Amphibious. The dragon can breathe air and water.

Assassinate. During its first turn, the dragon has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the dragon scores against a surprised creature is a critical hit.

Evasion. If the dragon is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the dragon instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Sneak Attack. Once per turn, the dragon deals an extra 14 (4d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the dragon that isn't incapacitated and the dragon doesn't have disadvantage on the attack roll.

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 7 (2d6) poison damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Poison Breath (Recharge 5–6). The dragon exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.

Alter Shape. The dragon magically transforms into a flying snake, or an assassin and retains its alignment, damage immunities, hit points, and Hit Dice, as well as its Intelligence, Wisdom, and Charisma scores. This transformation ends if the dragon is reduced to 0 hit points or if it uses another bonus action to end it.