Some of the scariest films I've ever seen are horror movies that feature demonic possession. Those that immediately come to mind for me include The Exorcist(1973), Amityville 2-The Possession(1982) and Beyond The Door(1974). Early next year on January 6th Paramount is releasing The Devil Inside, directed by William Brent Bell. Judging by the trailer it looks promising. Check out the trailer below: