Rob Halford has also performed as the vocalist for Black Sabbath at three shows. He filled in for Ronnie James Dio for two shows on November 14 and November 15, 1992, following the end of the Dehumanizer tour, when Dio refused to participate in the band as an opening act for original vocalist Ozzy Osbourne. On August 26, 2004, Halford filled in for Osbourne for an Ozzfest show in Camden, New Jersey, since Ozzy could not perform due to bronchitis.http://en.wikipedia.org/wiki/Rob_Halford