“The Lament of Tristan” is an anonymous 14th century Italian piece that I play on my 26 string lever harp.
Wikipedia has this to say about the legend of Tristan and Iseult:
Tristan and Iseult is a tale made popular during the 12th century through Anglo-Norman literature, inspired by Celtic legend, particularly the stories of Deirdre and Naoise and Diarmuid Ua Duibhne and Gráinne. It has become an influential romance and tragedy, retold in numerous sources with many variations. The tragic story is of the adulterous love between the Cornish knight Tristan (Tristram) and the Irish princess Iseult (Isolde, Yseult, etc.). The narrative predates and most likely influenced the romance of Lancelot and Guinevere in the Matter of Britain and has had a substantial impact on Western art, the idea of romantic love, and Western literature since it first appeared in the 12th century. While the details of the story differ from one author to another, the overall plot structure remains much the same.
There are two main traditions of the Tristan legend. The early tradition comprised the French romances of two poets from the second half of the twelfth century, Thomas of Britain and Béroul. Later traditions come from the Prose Tristan (c. 1240), which was markedly different from the earlier tales written by Thomas and Béroul. The Prose Tristan became the common medieval tale of Tristan and Iseult that would provide the background for the writings of Sir Thomas Malory, the English author, who wrote Le Morte d’Arthur (c. 1469).