Walgan, Modred's Brother, Geoffrey of Monmouth Text

Geoffrey of Monmouth's Historia Regum Brittanniae, "history of the kings of Britain," contains one of the most comprehensive versions of the Arthurian legend; though it is no longer considered valuable as a history, it is still an extremely valuable piece of medieval literature. It is easily found in translation; an ebook can be found at https://books.google.com/books?id=FUoMAAAAIAAJ. For the Latin, an ebook is available at https://books.google.com/books?id=LBw2AAAAMAAJ.