Proper noun
Lyonesse
(mythology) A kingdom mentioned in Arthurian legend, consisting of a long strand of land stretching from Land's End at the southwestern tip of Cornwall, England to what is now the Isles of Scilly in the Celtic Sea portion of the Atlantic Ocean. It was considered lost after being swallowed by the ocean in a single night.
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. Source: Internet