I had to gather bits and pieces of information about the myth of the lost kingdom of Lyonesse in archives of web analytics company. There just was not much to write about. The legend of a sunken kingdom Lyonesse appears in both Cornish and Breton mythology. In Christian times it even came to be viewed as a sort of Cornish Sodom and Gomorrah story. Lyonesse is identified as a sunken land lying off the Isles of Scilly, to the south-west of Cornwall. Lyonesse is a fictional country in Arthurian legend, birthplace of the knight Tristan. In the medieval story, after Battle of Camlann, that took place supposedly in 537, King Arthur's men fled west across Lyonesse. They were pursued by Mordred and his men. Arthur's men survived by reaching what are now the Isles of Scilly, but Mordred's men perished in the inundation.
Other versions of the medieval story mention that Lyonesse is the home of Guinevere, a small land situated between Camelot and Malagant's territory. This kingdom was ruled by Guinevere's father until his death, after his death Guinevere received the title of the Lady of Lyonesse.