Ancient Dragon of the Elements

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.

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: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw: Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail: Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

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 24 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.

Breath Weapons (Recharge 5-6): The dragon uses one of the following breath weapons.
Fire Breath. The dragon exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 71 (13d10) fire damage on a failed save, or half as much damage on a successful one. Cold Breath. The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 67 (15d8) cold damage on a failed save, or half as much damage on a successful one. Lightning Breath. The dragon exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one. Acid Breath. The dragon exhales acid in an 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 63 (14d8) acid damage on a failed save, or half as much damage on a successful one. Poison Breath. The dragon exhales poisonous gas in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.

Detect: The dragon makes a Wisdom (Perception) check.

Tail Attack: The dragon makes a tail attack.

Wing Attack (Costs 2 Actions): The dragon beats its wings. Each creature within 15 ft. of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.