Lyonesse is a country in Arthurian legend, particularly in the story of Tristan and Iseult. Said to border Cornwall, it is most notable as the home of the hero Tristan, whose father was king. In later traditions, Lyonesse is said to have sunk beneath the waves some time after the Tristan stories take place, making it similar to Ys and other lost lands in medieval Celtic tales, and perhaps connecting it with the Isles of Scilly.
Lyonesse in Arthurian legend
In medieval Arthurian legend, there are no references to the sinking of Lyonesse, because the name originally referred to a still-existing place. Lyonesse is an English alteration of French Léoneis or Léonois, a development of Lodonesia, the Latin name for Lothian in Scotland. Continental writers of Arthurian romances were often puzzled by the internal geography of Great Britain; thus it is that the author of the French Prose Tristan appears to place Léonois contiguous, by land, to Cornwall. In English adaptations of the French tales, Léonois, now "Lyonesse", becomes a kingdom wholly distinct from Lothian, and closely associated with the Cornish region, though its exact geographical location remained unspecified. The name was not attached to Cornish legends of lost coastal lands until the reign of Elizabeth I of England. However, the legendary lost land between Land's End and Scilly has a distinct Cornish name: Lethowsow. This derives from the Cornish name for the Seven Stones Reef, on the reputed site of the lost land's capital and the site of the notorious wreck of the. The name means 'the milky ones', from the constant white water surrounding the reef. Alfred, Lord Tennyson's Arthurian epic Idylls of the King describes Lyonesse as the site of the final battle between Arthur and Mordred. One passage in particular references legends of Lyonesse as a land fated to sink beneath the ocean:
Then rose the King and moved his host by night And ever pushed Sir Mordred, league by league, Back to the sunset bound of Lyonesse— A land of old upheaven from the abyss By fire, to sink into the abyss again; Where fragments of forgotten peoples dwelt, And the long mountains ended in a coast Of ever-shifting sand, and far away The phantom circle of a moaning sea.
Deriving from a false etymology of Lyonesse, as the 'City of Lions', it was said in some later traditions to be the capital of the legendary kingdom, situated on what is today the Seven Stones Reef, some eighteen miles west of Land's End and eight miles north-east of the Isles of Scilly.
Analogues in Celtic mythology
The legend of a sunken kingdom appears in both Cornish and Breton mythology. In Christian times it came to be viewed as a sort of Cornish Sodom and Gomorrah, an example of divine wrath provoked by unvirtuous living. There is a Breton parallel in the tale of the Cité d'Ys or Ker Ys, similarly drowned as a result of its debauchery with a single virtuous survivor escaping on a horse, in this case King Gradlon. The Welsh equivalent to Lyonesse and Ys is Cantre'r Gwaelod, a legendary drowned kingdom in Cardigan Bay. Not too dissimilarly, the Gaelic otherworld Tír na nÓg was often conceived of as mystical place that had to be reached via a sea voyage, though it lacks an inundation myth. It is often suggested that the tale of Lyonesse represents an extraordinary survival of folk memory of the flooding of the Isles of Scilly and Mount's Bay near Penzance. For example, the Cornish name of St Michael's Mount is Karrek Loos y'n Koos - literally 'the grey rock in the wood'. Cornish people around Penzance still get occasional glimpses at extreme low water of a sunken forest in Mount's Bay, where petrified tree stumps become visible. The importance of the maintenance of this memory can be seen in that it came to be associated with the legendary Brython hero Arthur, although the date of its inundation is actually c. 2500 BC.
Lyonesse was also an inspiration for Westernesse, also called Númenor, a realm which sank beneath the sea in J. R. R. Tolkien's stories associated with Middle-earth.
The manga and anime Seven Deadly Sins takes place in a fantasy version of medieval Brittania, and the main plot revolves around the kingdom of Liones.