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.

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Web.

Rend. Melee Weapon Attack:  +12, reach 10 ft. Hit: 16 (2d8 + 7) Slashing damage plus 2 (1d6) Acid damage.

Acid Breath (Recharge 5–6). Dexterity Saving Throw: DC 20, each creature in a 60-foot-long, 5-foot-wide Line. Failure: 54 (10d10) Acid damage. Success: Half damage. 

Embrittling Breath. The dragon targets each nonmagical metal object - armor or weapon- worn or carried by a creature in a 60 foot Cone. Dexterity Saving Throw: DC 20, creatures with said objects. Failure: The object takes a -1 penalty tot he AC it offers (armor) or to its attack rolls (weapon). Armor is destroyed if the penalty reduces its AC to 10, and the weapon is destroyed if the penalty reaches -5. The penalty can be removed by casting the Mending spell on the armor or weapon. Nonmagical, inanimate metal objects within the area take 10 Acid damage.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 20):

At Will: Detect Magic, Grease, Web, 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)
1/Day: Gaseous Form, Summon Dragon (Acid version)

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.

Liquid Slink. The dragon moves up to half its fly speed in any direction, unimpeded by difficult terrain. During this movement, the dragon is considered a Small creature. The dragon can't take this action again until the start of its next turn.

Ravage Metal. The dragon chooses a 10 foot Sphere within 60 feet of itself. Every nonmagical metal object within the sphere that isn't being worn or carried is destroyed. The dragon can't take this action again until the start of its next turn.

Headbutt. The dragon makes one Rend attack.