Ancient Gold Dragon

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) Spellcasting to cast Guiding Bolt (level 4 version) or (B) Weakening Breath.

Rend Melee Attack Roll: +17 to hit, reach 15 ft. Hit: 19 (2d8 + 10) Slashing damage plus 9 (2d8) Fire damage.

Fire Breath (Recharge 5-6) Dexterity Saving Throw: DC 24, each creature in a 90-foot Cone.

Spellcasting The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 24, +16 to hit with spell attacks):
At Will: Detect Magic, Guiding Bolt (level 4 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)
1/Day Each: Flame Strike (level 6 version), Word of Recall, Zone of Truth

Weakening Breath Strength Saving Throw: DC 24, each creature that isn't currently affected by this breath in a 90-foot Cone.

Preamble 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.

Banish Charisma Saving Throw: DC 24, one creature the dragon can see within 120 feet.

Guiding Light The dragon uses Spellcasting to cast Guiding Bolt (level 4 version).

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