Amphibious. The dragon can breathe air and water.

Legendary Resistance (4/Day, or 5/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 (A) Repulsion Breath or (B) Spellcasting to cast Guiding Bolt (level 2 version).

Rend. Melee Attack Roll: +16, reach 15 ft. Hit: 18 (2d8 + 9) Slashing damage plus 9 (2d8) Lightning damage.

Lightning Breath (Recharge 5–6). Dexterity Saving Throw: DC 23, each creature in a 120-foot-long, 10-foot-wide Line. Failure: 82 (15d10) Lightning damage. Success: Half damage.

Repulsion Breath. Strength Saving Throw: DC 23, each creature in a 30-foot Cone. Failure: The target is pushed up to 60 feet straight away from the dragon and has the Prone condition.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 22, +14 to hit with spell attacks):

At Will: Detect Magic, Guiding Bolt (level 2 version), 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), Speak with Animals, Thaumaturgy
1/Day Each: Detect Thoughts, Control Water, Scrying, Water Breathing

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.

Guiding Light. The dragon uses Spellcasting to cast Guiding Bolt (level 2 version).

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.

Thunderclap. Constitution Saving Throw: DC 22, each creature in a 20-foot-radius Sphere centered on a point the dragon can see within 120 feet. Failure: 13 (3d8) Thunder damage, and the target has the Deafened condition until the end of its next turn.