The ancient dragon turtle is a mythical creature, as large as a decently-sized island, reaching a length of one mile or more. It is fair to say that ancient dragon turtles might very well be the largest living creatures in existence, at least on the Material Plane - their mouth alone is large enough to swallow a kraken whole. An ancient dragon turtle does not just stand out due to its size, it also is of an incredible age, having seen and survived ages where men and elves were not yet part of the world. A dragon turtle wyrmling can be considered a young dragon turtle at the age of a few thousand years, which is the same age a true dragon would aready be considered ancient, and it takes a few tens of thousands of years to grow into an adult dragon turtle. One can only imagine how long it would take for an adult dragon turtle to grow into an ancient one.

Ancient dragon turtles usually slumber for years, decades, centuries or even longer - during that time, they float at the ocean's surface, they essentially become large, mountainous swimming islands, giving nature a chance to conquer their shell. And sometimes, even people start settling on the back of a dragon turtle, mistaking it for an island - until, in a catastrophic event, the island reveals its true nature and awakens.

World-Bearers. Ancient dragon turtles are so large and slumber for so long, that an entire self-sufficient ecosystem can develop on their hard back, including forest, moss, grass and various kinds of animals, with large sea-birds nesting at the highest points of the shell. The back of a dragon turtle is large enough for adventurers to live on it and to explore it like a normal island. In combat, characters may need to move across the shell to reach the vulnerable head and limbs of the turtle, while overcoming various challenges imposed by the unusual and in some places difficult-to-traverse terrain.
Similarly, a dragon turtle that spent a long time submerged might have become the base of a great coral reef, with its shell becoming home to unique, beautiful forms of life from the deeps. Whatever kind of world a dragon turtle bears, from time to time, when the turtle awakens, it undergoes great change.

Dungeons Within.  The shell of a dragon turtle, while solid on the outside and sturdy enough to protect the dragon turtle against any damage, might actually be home to cave systems or other kinds of dungeons. Maybe someone mistook it for an island thousands of years ago and built a barrow for a fallen legend, full of treasure, in its shell? Maybe other creatures lair within the turtle's shell, not aware of this island actually being a living creature by itself?

Devourer of Ships. An angered dragon turtle might swallow an adventurer or even a whole ship with its bite attack. In this case, the swallowed character(s) could either be subjected to the usual rules for swallowing (blinded, restrained, total cover, taking 36 (8d8) acid damage at the start of each of their turns), or the DM could decide the turtle's inside is spacious enough to have the character navigate the dragon turtle's massive body as if it was a giant, living dungeon. They might encounter unique hazards in there, such as stomach acid, old shipwrecks - and even monsters that live there naturally or have been swallowed whole by the dragon turtle.

An ancient dragon turtle's lair is usually an underwater cave or a coral reef situated deep beneath the waves where the ocean reaches its greatest depths and is accommodated by submarine volcanic activities including hydrothermal deep-sea vents and even volcanoes erupting glowing hot lava into the cold sea-water. The area around an ancient dragon turtle's lair is a desirable target for natural explorers, as deep-sea volcanism creates a unique habitat, which allows for unusual and exceptionally rare and almost alien forms of life to manifest there. However, considering the immense depths in which these lairs are found and the dangers that loom there, the journey to a dragon turtle's cave and the volcanic fields around it is a dangerous expedition, only suited for skilled adventurers who have the training and magic necessary to survive in these immense depths.

In terms of cave structure and layout, a dragon turtle's lair usually is eerily similar to the seaside caves preferred by topaz and bronze dragons, although of course of an entirely different scale. An ancient dragon turtle is largely unconcerned with the design of its lair and rarely if ever works to improve it, as its primary concern is the lair's size - it must be large enough for a creature that by itself is the size of an island, and it must provide enough space for that creature's hoard. Ancient dragon turtles hoard treasure in their lair like any other dragons, and their hoards, as ancient as their owners, can tell stories of long-forgotten ages. Not just gold, jewellery and magic items are found in such a hoard, but also entire ships and even artifacts from the very beginning of the world - artifacts that were not created by elven- or humankind, but by whatever was there before.

Lair Actions

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:

• Steam Eruption. 1d4 hydrothermal vents erupt clouds of steam and smoke, with each cloud causing an area of a 90-feet cone to be heavily obscured until initiative count 20 on the next round. Each creature in at least one of these areas must make a DC 20 Constitution saving throw, taking 36 (8d8) fire damage on a failed save, or half as much damage on a successful one. Being underwater doesn't grant resistance against this damage.
• Entangle. Strands of kelp and tentacles from strange deep-sea forms of life reach out within a 20-foot-tall cylinder with a radius of 60 feet centered on a point on a solid surface to ensnare any intruders to the lair. Each creature within the area must make a DC 20 Dexterity saving throw or be restrained until the end of its next turn.
• Deep-Sea Quake. An earthquake hits the lair, causing it to shake violently. The water grows turbulent and pieces of debris break loose. Each creature within the lair must make a DC 20 Strength saving throw. On a failure it takes 26 (4d12) bludgeoning damage and is knocked prone, on a success it takes half as much damage without other effects. In addition, the whole lair counts as difficult terrain until initiative order 20 on the next round.
• Submarine Lava Flow. A fissure opens and erupts lava, covering an area along the grounds that is up to 600 feet long and up to 60 feet wide. Each creature that starts its turn within that area takes 55 (10d10) fire damage. Being underwater doesn't grant resistance against this damage. The lava flow lasts for 1 minute or until the dragon turtle uses this lair action again.
• Toxic Fumaroles. 1d4 vents in the lair release toxic gas into the water. Each creature within 60 feet of at least one of these vents must make a DC 20 Constitution saving throw or be poisoned for 1 minute. While poisoned in this way, a creature is unable to breathe. A creature can repeat the saving throw at the end of each of its turns, ending the effect on a success.

Regional Effects

The region containing an ancient dragon turtle's lair can be transformed by its presence, resulting in one or more of the following effects:

• Strong currents try to push unwanted visitors away from the lair. These currents can be first encountered within 15 miles of the lair and become more common closer to the lair.
• The region within 10 miles of the lair is a field of geothermal deep-sea vents and sometimes even a showplace of submarine volcanic eruptions. This causes the water to reach temperatures of a hundred or more degrees in some places, while toxic gases released into the water by volcanic vents might make breathing difficult to impossible even for most water-breathing creatures.
• Within 10 miles of the lair, the fields of deep-sea vents and submarine volcanoes create a habitat for unusual deep-sea creatures and other strange forms of life ranging from tiny worms to gargantuan monsters, while most fish and other well-known common sea creatures are deterred by the environment or straight up unable to survive there.
• Within 5 miles of the lair, crevasses in the ocean floor form portals to the depths of the Elemental Plane of Water and to elemental border regions, where the Elemental Plane of Water and the Elemental Plane of Fire merge, allowing creatures of elemental water and of elemental fire to pass into the world to dwell nearby.