The Cornish island where – according to Chrétien de Troyes and the prose Tristan – Tristan fought and killed Morholt (Marhaus) in his first duel (Tristan was championing King Mark against Morholt, who had demanded a tribute from Cornwall). It was apparently visible from the mainland.
In La Tavola Ritonda, the battle takes place on an island called Sanza Avventura. Béroul names Saint Samson as a monastery in King Mark’s Cornwall, and, according to Geoffrey Ashe, an actual church named Saint Samson still exists in Cornwall, on a hillside in Golant. Presumably it lies between Britain and Ireland; there is an island called Saint Samson in the Scilly group.
See also
Saint Samson Church | The Legend of King Arthur
Sources
Tristan | Béroul, late 12th century
Erec | Chrétien de Troyes, late 12th century
Prose Tristan | 1230-1240