Amphibious.

The dragon turtle can breathe air and water.

Blessing of the Sea (Recharges after a Short or Long Rest).

If the dragon turtle would be reduced to 0 hit points, its current hit point total instead resets to 350 hit points, and it recharges its Steam Breath. Additionally, the dragon turtle can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 62,000 XP (124,000 XP total) for defeating the dragon turtle after its Blessing of the Sea activates.

Legendary Resistance (3/Day).

If the dragon turtle fails a saving throw, it can choose to succeed instead.

Unusual Nature.

The dragon turtle doesn't require food or drink.

Actions

Multiattack.

The dragon turtle makes one Bite or Tail attack and two Claw attacks.

Bite.

Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 15 (1d12 + 9) piercing damage plus 13 (2d12) lightning damage.

Claw.

Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) slashing damage.

Tail.

Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage. If the target is a creature, it must succeed on a DC 24 Strength saving throw or be knocked prone.

Steam Breath (Recharge 5–6).

The dragon turtle exhales steam in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 67 (15d8) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant resistance against this damage.

Legendary Actions

The ancient dragon turtle 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. The ancient dragon turtle regains spent legendary actions at the start of its turn.

Attack. The dragon turtle makes one Claw or Tail attack.
Move. The dragon turtle moves up to its speed. If the dragon turtle is swimming, this movement doesn't provoke opportunity attacks.
Boiling Aura (Costs 3 Actions). The dragon turtle radiates intense heat. Until the start of the dragon turtle's next turn, whenever a creature starts its turn within 20 feet of the dragon turtle, that creature must succeed on a DC 24 Constitution saving throw or take 40 (9d8) fire damage. Being underwater doesn't grant resistance against this damage.

Mythic Actions

If the dragon turtle's Blessing of the Sea trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The dragon turtle makes one Bite attack.
Armor of Storms (Costs 2 Actions). Lightning temporarily surrounds the dragon turtle, and it gains 40 temporary hit points until the start of its next turn. Until all these temporary hit points are gone, any creature that touches the dragon turtle or hits it with a melee attack takes 26 (4d12) lightning damage.

Lair Actions

As they are presented in the Monster Manual, dragon turtles don't have access to lair actions while in their lairs. At your discretion, you can give an adult or ancient dragon turtle access to lair actions, increasing its challenge rating by 1. On initiative count 20 (losing initiative ties), the dragon turtle can take one of the following lair actions; the dragon turtle can't take the same lair action two rounds in a row:

Blasting Current. A strong water current moves through the dragon turtle's lair. Each creature within 30 feet of the dragon turtle must succeed on a DC 15 Strength saving throw or be moved 30 feet in a direction of the dragon turtle's choice. On a success, the creature is moved 10 feet instead.
Entangling Kelp. Strands of spectral kelp reach out to ensnare creatures in a 20-foot-radius sphere centered on a point in the lair that the dragon turtle can see. Each creature in that area must succeed on a DC 15 Strength saving throw or be restrained until the end of its next turn.
Steam Eruption. Steam erupts in a 15-foot cone from a point on the ground the dragon turtle can see within 120 feet of it. Each creature in that area must make a DC 15 Constitution saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant creatures resistance to this damage.

Regional Effects

The region containing a dragon turtle's lair can be transformed by its presence, creating one or more of the following effects:

Diverting Currents. Underwater currents push unwanted visitors away from the lair. While swimming in these currents, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), even if a creature has a swimming speed.
Elemental Portals. Crevasses on the ocean floor within 1 mile of the dragon turtle's lair form two-way portals to the Elemental Plane of Water, allowing creatures of elemental water into the world to dwell nearby.
Hot Water. Geothermal vents heat the water within 6 miles of the dragon turtle's lair to 100 degrees Fahrenheit.

If the dragon turtle dies, these effects fade over the course of 1d10 days.

Ancient Dragon Turtle