Not only did William Peter Blatty write the second sequel to this film, he directed it as well
The Exorcist, written in 1971, is his most well-known novel; he also wrote the 
screenplay for the film adaptation, for which he won an Academy Award, and 
wrote and directed the sequel The .... Blatty had no involvement in this first sequel 
and his own follow-up ignored it... [DOC] [TLE] The Exorcist III - Wikipedia [PAR] The Exorcist III is a 1990 American supernatural horror film written and directed 
by William Peter Blatty ...