The celebrated medievalist William Morris was among the first writers to combine supernatural elements with worldbuilding. He created the precursors of modern fantasy fiction, and both C. S. Lewis and J. R. R. Tolkien acknowledged his influence on their writing. In his final novel, Morris draws upon Icelandic lore to tell a tale of two lovers, Osberne and Elfhild, separated by a broad river. When Elfhild vanishes from the riverbank, Osberne takes up...