Holmes is visited by a masked gentleman introducing himself as Count Von Kramm, an agent for a wealthy client, but Holmes quickly deduces that he is in fact Wilhelm Gottsreich Sigismond von Ormstein, Grand Duke of Cassel-Felstein, the hereditary King of Bohemia. The King admits this, tearing off his mask. (Actually, the Habsburg Emperors were also Kings of Bohemia and there was no separate dynasty; Doyle chose to place an imaginary king at an existing country, rather than create a whole imaginary country such as Ruritania).
