early form: Liones
possibly from Leonais
, French name for Lothain
Then rose the King and moved his host by night,
And ever push'd 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.
Tennyson, Idylls of the King
The mythical land which extended Cornwall beyond the Isles of Scilly into the Celtic Sea. The capitol was the City of Lions (a mistake for Caerllion?), now said to be the Seven Stones in the area of Tregva, an outcropping of rock from which fishermen have claimed to pull out window frames and doors.
It is said to be the birthplace of Tristan, ruled by his father King Meliodas. Alfred Lord Tennyson makes it the setting for the Last Battle between Arthur and Mordred.
Legend has it that the land sunk beneath the sea in 1089 (others say on November 11, 1099) when a massive storm hit the land; the only survivor was a Trevelyan, of Basil near Launceston, who outroad the waves on his white horse. He founded the Trevelyan family of Cornwall, whose arms show a horse riding on the sea.
There are said to be 140 churches under the waves, which can still be heard.
The first attempt to make Lyonesse a real place is in the itinerary of one William Worcestre in 1478, and later in Richard Carew's Survey of Cornwall (1602) and William Camden's Brittannia (1586). All refer to it as off the coast of Cornwall.
Lyonesse is not off the coast of Cornwall, nor was it ever. Instead, there is the land of Lethowstow, which is the Cornish name for the sunken kingdom. The word Lyonesse is probably from the French Leonais, their name for Lothian, Scotland.
Why would Scotland and Cornwall be confused? The answer likely lies in the figure of Tristan--or really the Pictish king Drust V, son of Tallorcan III, known in Welsh as Drystan ap Tallwch and identified with the famous nephew of King March ap Meirchion. Conveniently, Lothian was inhabited by the Picts.
At some point, the Leonais of Scotland may have been confused with the Leonais of Brittany, which is (relatively) close to Cornwall. But why? And why place Drust in Cornwall in the first place?
At Castle Dore in Fowey, Cornwall, there is a stone monument with the legend Drustanus Hic Lacit Cunomori Filius "Here lies Drustan, son of Cunomori". Or, in Brittonic names, Drystan and Cynvawr. Cynvawr--"great hound"--is also called Cynfarch, son of Meirchion. March, of course, is also son of Meirchion, and thus would be Drustan's uncle. Moreover, Cynfarch was the king of Rheged, in the north of Britain (and was the father of Urien). Though a 9th century "Life of St. Paul Aurelian" by the Breton monk Wrmonoc names a King Marcus also known as Quonomorius. This is likely a mistake, confusing Cynfarch with his brother March.
Ultimately, there seem to be two Tristans: the son of Cynfarch of Rheged, who somehow made it to Cornwall, and the eighth century Pictish king. Both belong to the north, but one evidently came to the south. It is the confusion between the two that lead to the conflating of Lothian with Lethowstow into the mythical sunken land of Lyonesse.
Another Myth? Or Possibly Real?
This land may have existed--in Roman times, the Isles of Scilly were refered to as Siluram Insulam--a single island. It's thought that the isles (some of which can be walked to, one island to another, in low tide) were once one large, wooded isle. Moreover, there is a drowned forest in Mount's Bay.
Celtic myth has a number of lost lands which have sunk beneath the sea: Cantre'r Gwaelod, Ys, Tir fo Thoinn. Maybe there is some basis in fact; after all, the Suffolk town of Dunwich has been swallowed by the sea, why not these? Certainly not as recent as the Dark Ages, but perhaps in some Bronze Age era? Sometimes the sea gives up its secrets.