The legend of Tristan and Iseult is an influential romance and tragedy. The tragic story is of the adulterous love between the Cornish knight Tristan and the Irish princess Iseult. The narrative predates and most likely influenced the Arthurian romance of Lancelot and Guinevere, and has had a substantial impact on Western art and the idea of romantic love and literature.

""My lords, if you would hear a high tale of love and of death, here is that of Tristan and Queen Iseult; how to their full joy, but to their sorrow also, they loved each other, and how at last they died of that love together upon one day; she by him and he by her."

""Long ago, when Mark was King over Cornwall, Rivalen, King of Lyonesse, heard that Mark's enemies waged war on him; so he crossed the sea to bring him aid; and so faithfully did he serve him with counsel and sword that Mark gave him his sister Blanchefleur, whom King Rivalen loved most marvelously. . . .""

This book by M. Joseph Bedier, originally in French "Le Roman de Tristan et Iseut" was crowned by the French Academy. It was translated and published in English in 1913 by George Allen & Company, Ltd. of London, and it's a heck of a book; if you have an interest in things Arthurian or in French literature of the period that gave us things like impressionism, you need to read this now.