Gargantuan Dragon, Neutral
Armor Class 20 Initiative +6 (16) Hit Points 356 (23d20 + 115) Speed 20 ft., Swim 50 ft.
Saving Throws Str +7, Dex +0, Con +11, Int +0, Wis +7, Cha +1
Damage Resistances Fire
Senses Darkvision 120 ft.; Passive Perception 11
Languages Draconic, Primordial (Aquan)
Challenge Rating 17
The dragon can breathe air and water.
The dragon makes three Bite attacks. It can replace one attack with a Tail attack.
Melee Attack Roll: +13, reach 15 ft. Hit: 23 (3d10 + 7) Piercing damage plus 7 (2d6) Fire damage. Being underwater doesn't grant Resistance to this Fire damage.
Melee Attack Roll: +13, reach 15 ft. Hit: 18 (2d10 + 7) Bludgeoning damage. If the target is a Huge or smaller creature, it has the Prone condition.
Constitution Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: 56 (16d6) Fire damage. Success: Half damage. Failure or Success: Being underwater doesn't grant Resistance to this Fire damage.