Gwyddno Garanhir was the supposed ruler of a sunken land off the coast of Wales, known as Cantre'r Gwaelod.
His chief fortress was said to have been Caer Wyddno (English: the Fort of Gwyddno), located somewhere to the north-west of modern-day Aberystwyth.
One day the keeper of the floodgates, Seithenyn, was drunk and failed to close them, with the result that the sea rushed in and covered the land.
Stories of the drowned lands of Gwyddno appear to have arisen from the identification of natural underwater ridges as the remains of sea walls.
In Susan Cooper's The Dark Is Rising series, Gwyddno is the king of the Lost Lands in Silver on the Tree.