The Lorekeeper's Tome

The Dragon Turtle is one of the great horrors of the deep sea: an immense armored beast, its shell vast enough that sailors have made landfall on a sleeping one believing it an island, only to be carried under when it woke and dove. It breathes a scalding cloud of steam that boils the sea around it and parboils any crew unlucky enough to be near, and its shell turns aside ballista and spell alike. Dragon Turtles are slow, ancient, and territorial, ruling stretches of deep ocean as a dragon rules a mountain, and a ship that strays into one's waters rarely strays back out.