- "Blessed by the Drowned God, the Grey King fought and slew Nagga the great Sea Dragon and took her fire for his own."
- ―Theon Greyjoy
In the books
Sea dragons are possibly mythical creatures of the sea. It is unknown if sea dragons are related to dragons. Some maesters question their existence since they have not been seen in millennia. Believers think they still exist deep in the Sunset Sea far from the sight of man.
According to ironborn legend, Nagga was the first and mightiest sea dragon, a creature which fed on krakens and leviathans. She was slain by the Grey King, however, and her bones formed part of the Grey King's Hall. The alleged ruins of his hall still exist on Old Wyk, and Nagga's Bones are the traditional location where each Kingsmoot is held.
It was a little unclear in the main novels if "Nagga's Bones" were actual bones or some natural stone pillar formation: the World of Ice and Fire sourcebook (2014) confirmed that they are indeed the fossilized bones of some ancient animal of massive size - though whether they are from a sea dragon, a huge whale, or some other kind of creature is anyone's guess.