... The last Umayyad Caliph Hishám II, had vanished mysteriously:  it was generally supposed that, after escaping from Cordova when that city was stormed by the Berbers (1013 A.D.), he fled to Asia and died unknown; but many believed that he was still alive.  Twenty years after his disappearance there suddenly arose a pretender, named Khalaf, who gave out that he was the Caliph Hishám.  The likeness between them was strong enough to make the imposture plausible... (A Literary History of the Arabs, Nicholson, 1907)