Amphibious. The dragon turtle can breathe air and water.

Colossal. The dragon turtle is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever the dragon turtle makes a weapon attack, regardless of whether the attack hits or misses, each creature within 40 feet of the attack's target other than the target or the dragon turtle itself must succeed on a DC 20 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The dragon turtle's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

False Appearance. While the dragon turtle remains motionless at the ocean's surface, it is indistinguishable from a normal, mountainous island.

Heavy Sea. If the dragon turtle is awake, the sea within 600 feet of it becomes rough. The water within that area counts as difficult terrain. Characters might experience strong, unexpected currents and high waves, resulting in ships capsizing or breaking apart and other dangers, on DM's discretion.

Indestructible Shell. The dragon turtle's shell is treated as difficult terrain and is indestructible. The dragon turtle is immune to any attack, spell or other effect that only targets its shell or only includes its shell in its area of effect. To affect or damage the dragon turtle, an attack, spell or other effect must target its head or limbs, or include them in its area.

Inflexible. The dragon turtle is incapable of targeting a creature on its shell with an attack or with its breath weapon. It also automatically fails any dexterity saving throw.

Legendary Resistance (5/day). If the dragon turtle fails a saving throw, it can choose to succeed instead.

Lightning Absorbtion. When the dragon is subjected to lightning damage, it takes no damage and instead regains an amount of hit points equal to the lightning damage.

Moving Island. The dragon turtle ignores difficult terrain and can enter spaces occupied by other creatures. Depending on whether the dragon turtle is moving in the water or on land, the following effects happen to creatures in its path:

• If the dragon turtle enters another creature's space in water, that creature is harmlessly pushed aside by the current. A creature that comes within reach of the shell can also use its reaction to make a DC 20 Strength (Athletics) check to latch onto the shell.
• If the dragon turtle is moving on land, the first time on a turn it enters the space of another creature, that creature must make a DC 20 Dexterity saving throw or take 52 (8d12) bludgeoning damage and be knocked prone.

Multiattack. The dragon turtle uses its Wild Current and makes three attacks: one with its bite or tail and two with its claws. If it is swimming at the ocean's surface, it can then use its Titan's Dive.

Bite. Melee Weapon Attack: +19 to hit, reach 60 ft., one target. Hit: 62  (8d12 + 10) piercing damage plus 26 (4d12) lightning damage. The target is unable to use any reactions until the start of its next turn and the dragon turtle can grapple it as part of that attack.

Claw. Melee Weapon Attack: +19 to hit, reach 60 ft., one target. Hit: 46 (8d8 + 10) slashing damage. If the target was not underwater, it must make a DC 20 Strength saving throw or is pushed up to 3d6 × 10 feet away from the dragon turtle, landing prone.

Tail. Melee Weapon Attack: +19 to hit, reach 80 ft., one target. Hit: 54 (8d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Titan's Dive. The dragon turtle uses all its movement to dive beneath the water's surface. As the water swallows it and crashes together over it, all creatures on the dragon turtle's shell and all creatures within 600 feet of the dragon turtle in the water must make a DC 20 Strength saving throw. On a failure, they take 52 (8d12) bludgeoning damage and suffer the following effects:

• Creatures on the dragon turtle's shell are caught in the current and washed off the shell. They end up in random spaces along the path the dragon turtle took.
• Creatures that were within 600 feet of the dragon turtle in the water are caught in the current and are drawn up to 3d6 × 10 feet towards the location the dragon turtle was at at the start of its turn.

Creatures who succeed on the saving throw take half damage without other consequences. If they were on the turtle's shell, they can continue to cling to it.

Wild Current. The dragon turtle, while in water, creates a strong current in a 600-foot line that is 60 feet wide. All creatures caught in that current must make a DC 20 Strength saving throw. On a failure, they are pulled up to 3d6 × 10 feet closer to the dragon turtle or pushed the same distance away (the dragon turtle's choice) and are knocked prone.

Steam Breath (Recharge 5–6). The dragon turtle exhales steam in a 1200-foot cone. Each creature in that area must make a DC 20 Constitution saving throw, taking 72 (16d8) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant resistance against this damage. Until the start of the dragon turtle's next turn the area covered by the breath is heavily obscured by steam or bubbles in the water.

Armor of Storms. Lightning temporarily surrounds the dragon turtle, and it gains 60 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, starts its turn while in cotact with the dragon turtle or hits it with a melee attack takes 26 (4d12) lightning damage.

Emerge. The dragon turtle emerges from its shell after having used its Withdraw reaction.

Lightning Rod. When a creature within 600 feet of the dragon turtle deals lightning damage and the dragon turtle can see that creature, it can use its reaction to draw in the lightning. In this case, the creature targets the dragon turtle with the lightning damage, no matter the effect's original range.

Withdraw. If the dragon turtle takes a critical hit, it can use its reaction to withdraw into its shell. In this state it is immune to all damage unless the players find a way to get into its shell to reach its head or limbs, and it cannot move or take actions until it uses a bonus action on its turn to emerge again.

The ancient dragon turtle can take 5 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.

Rush. The dragon turtle moves up to its speed. If it is submerged in water, all creatures on its shell must make a DC 20  Strength saving throw or be washed off the shell by the current.

Lightning Strike.  This legendary action can only be used if the dragon turtle's Armor of Storms is active. A lightning bolt strikes a point within 600 feet of the dragon turtle. All creatures within 10 feet of that point must make a DC 20 Dexterity saving throw, taking 36 (8d8) lightning damage on a failure or half as much on a success

Maelstrom (Costs 2 Actions). The dragon turtle targets a point within 1200 feet of it that is within water. A maelstrom with a radius of 60 feet at the top and a depth of 60 feet appears at that point. Until the start of the dragon turtle's next turn, this area is difficult terrain, and any creature that starts its turn there must succeed on a Strength saving throw or take 36 (8d8) bludgeoning damage and be pulled into the center of the maelstrom, where it becomes restrained.

Boiling Aura (Costs 3 Actions, recharge 5–6).The dragon turtle radiates intense heat. The water within 120 feet of the dragon turtle starts boiling, causing the area to be heavily obscured by steam or bubbles in the water. The effects last until the start of the dragon turtle's next turn. Until then, whenever a creature starts its turn within 120 feet of the dragon turtle, that creature must succeed on a DC 20 Constitution saving throw or take 54 (12d8) fire damage. Being underwater doesn't grant resistance against this damage.