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-Léon area of Brittany), are also linked to the name Lyonesse, and it may be that Tristan came from one of these.
In Tennyson's poem Morte D'Arthur, King Arthur's final battle is set in Lyonness, the spelling of which matches Waugh's in Brideshead Revisited.
So all day long the noise of battle roll'd
Among the mountains by the winter sea;
Until King Arthur's table, man by man,
Had fall'n in Lyonness about their Lord.