William Peter Blatty, an author whose novel “The Exorcist” and his later screenplay for the 1973 film about a demonic possession became a phenomenon and stirred fierce public debate about the occult, died Jan. 12 at a hospital near his home in Bethesda, Md. He was 89. The cause was a form of multiple myeloma, said his wife, Julie Witbrodt Blatty. Mr. Blatty was a junior at Georgetown University when, in 1949, he became mesmerized by a Washington Post story detailing the alleged exorcism by a Jesuit priest of a 14-year-old boy from Mount Rainier, Md. who was believed to...