When he made his 1973 classic, The Exorcist, WILLIAM FRIEDKIN had never seen an exorcism, and for decades he wondered how close his film had come to reality. Last May, in Rome, the director was allowed to film as Father Amorth—a 91-year-old Vatican legend who would die just four months later—fought to expel Satan from an Italian woman. It began a chilling investigation into the phenomenon of possession