Maleagant

In that text, Melwas, king of the "Summer Country" (regnante in aestiua regione; a direct translation of the Old Welsh name for Somerset, Gwlad yr Haf), carries Guinevere (Gwenhwyfar) off to his stronghold of Glastonbury.

An early 12th-century monumental carving on the archivolt of Modena Cathedral in Italy contains a related scene, in which Arthur and his warriors besiege a castle where a character identified as Mardoc sits with Winlogee, presumably Guinevere.

[2] Maleagant (spelled Meliagant or Meliaganz) first appears under that name in Lancelot, the Knight of the Cart by Chrétien de Troyes, where he is said to be the son of King Bagdemagus, ruler of the otherworldly realm of Gorre (the Land of No Return), and brings the abducted Guinevere to his impenetrable castle out of his one-sided love for Arthur's wife.

Meleagan's island is perfectly square and its walls are made of crystal; there is a palace at each corner and a fountain wells up through a gilded copper horn at the center.

[3] Maleagant appears in modern retellings like Marion Zimmer Bradley's The Mists of Avalon and T. H. White's The Once and Future King (as Sir Meliagrance).