Amphibious. The dragon can breathe air and water.

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

Dissolve. When a creature, object, or area takes acid damage from the dragon's acidic mucus, it begins to dissolve. A dissolving creature takes 14 (4d6) acid damage at the start of its turn and cannot use reactions. Dissolving creatures continue dissolving for 3 (1d6) rounds, unless a creature takes an action to remove or neutralize the acid on itself, another, or a medium sized area.

Vitriolic Body. A creature that touches the dragon or hits it with a melee attack while within 5 feet of it takes 7 (2d6) acid damage.

Undead Fortitude. If damage reduces the dragon to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the dragon drops to 1 hit point instead

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) acid damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Acidic Mucus. Ranged Weapon Attack: +9 to hit, range 120 / 240 ft., one target. Hit: 18 (4d8) acid damage, and the target must succeed on a DC 17 Constitution saving throw, losing any resistance to acid damage for 1 minute and suffering the effects of the dragon’s dissolve trait

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 17 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one. The line also hits anything under it, as it spills and drops down.

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.

Move. The dragon flies up to half its speed.

Tail Attack. The dragon makes a tail attack.

Acidic Mucus (Costs 2 Actions). The dragon makes an acidic mucus attack.