The Imperial March is far too well-known to work for background music, but consider the "Emperor's Throne Room" piece from Return of the Jedi. It's this low, ominous, wordless chant, perfect for a noncombat encounter with a truly diabolic villain. (Although there might be a few places where the Imperial March theme crops up, don't remember offhand. How good are you at sound editing?)
For a more martial theme, "March of the Dead" from the Army of Darkness soundtrack is very good.
Are you looking for orchestral music or something in a more modern vein?