The Walkyre.
(Die Walküre) A musical drama in three acts.

APA Citation

Wagner, R. (1877). The Walkyre: (Die Walküre) A musical drama in three acts. New York: Theatre Ticket Office.

MLA Citation

Wagner, Richard, 1813-1883. The Walkyre: (Die Walküre) A Musical Drama In Three Acts. New York: Theatre Ticket Office, 1877.

Warning: These citations may not always be complete (especially for serials).