One of the strangest productions ever committed to celluloid, and the first feature with all dialogue recorded in Esperanto, this bizarre supernatural art-horror epic, hailed by Famous Monsters founder Forrest J. Ackerman as "the movie-watching event of a lifetime," actually benefits from the presence of a pre-Star Trek William Shatner, whose operatic style somehow conforms to the story's deranged logic. Shatner plays Marc, a man lost in the mythical land of Nomen Tuum where he comes under psychic attack from both the evil witch Kia (Allyson Ames) and the title demon (Milos Milos), who procures female souls for Satan. The filmmakers reportedly adopted the "universal language" of Esperanto to give the dialogue a mystical feel, but the end product may leave audiences wondering if the entire project is an elaborate put-on. Thoroughly strange, Incubus is certainly not without merit: the film's strength comes primarily from sumptuous location cinematography by Conrad Hall, who may have taken inspiration from the works of Ingmar Bergman and Akira Kurosawa. Believed lost, the only surviving negative of this oddity was eventually rescued from 30 years of oblivion and released to home video.
~ Cavett Binion, All Movie Guide