Lyonness, usually written Lyonesse, is the name given to a mythical land that once bordered Cornwall, before being submerged beneath the sea. The Isles of Scilly are sometimes said to be the last remnant of it.
It is thought to be the native land of Tristan, the knight in the story of Tristan and Iseult. However two other places, Leoneis (Lothian, in Scotland) and Leonois (in the Saint-Pol-de-L