Use the adjective demoniac to describe something or someone who seems to be possessed by a demon, like the demoniac rantings of the actors portraying scary creatures at the haunted house.
To pronounce demoniac, accent the second syllable: "de-MOE-nee-ac." The word has a somewhat confusing origin. The Latin word daemon means "spirit" and comes from the Greek word daimon, meaning "lesser god" or "guiding spirit." The Christian Greeks, however, translated it as "god of the heathen" and "unclean spirit." It is this negative connotation that is connected to demoniac: "like a demon."