Wagner was a German composer whose astonishing works made him one of the most influential figures in the history of Western music. He revolutionized the 19th-century idea of opera, seeing it as a wholly new art form in which musical, poetic, and scenic elements should come together through such devices as the leitmotif—a recurring melodic phrase used to illustrate a character or idea. His operas include Tristan and Isolde, as well as his masterpiece, a four-opera cycle known as what? Discuss