On the fabled isle of Nomen Tuum is a legend that the waters there have restorative properties which attracts various travellers in search of its magic, although most of them are corrupt. Acknowledging this is the presence of a succubus or two, including one, Kia (Allyson Ames), who likes to lure men to their deaths in the surrounding ocean, as she does today with a chap who drinks of the waters and is entranced by her. She leads him on to the shoreline, and as he stumbles about under the influence she holds his head under the water until he drowns. However, Kia might have met her match with love...
Incubus was the brainchild of Leslie Stevens, most famous as a television producer: The Outer Limits was his best known work in the genre. He was apparently highly impressed by the invented language of Esperanto, so much so that he wrote a script that was entirely based around it, and if this film is recalled for anything it is that the cast speak their every line in that dialect. Esperanto was intended to be a universal language, but it never really worked out, which is why you don't meet many people speaking it these days, but that was not all that did not work out about this production, as some believed it was cursed.
According to star William Shatner anyway, as the story goes that the cast and crew managed to enrage a hippy type who placed that curse upon them during filming. So it was that in spite of being well received, the prints of the film were lost for about thirty years, and that was not all the misfortune which was visited upon the moviemakers. For a start, Stevens' marriage to leading lady Ames was over within the year, but things got darker than that as the actor playing the titular Incubus, Milos Milos, murdered Mickey Rooney's estranged wife and killed himself in a still not adequately explained murder-suicide, the actress playing Shatner's sister, Ann Atmar, killed herself too, and the actress in the role of Ames' sister, Eloise Hardt, suffered the unsolved murder of her teenage daughter.
Whether this was down to a curse is of course debatable, as soon after Shatner went on to worldwide fame in Star Trek, cinematographer Conrad L. Hall's career was taking off in a significant way as well, and besides, Stevens continued to work in the industry with quite a few TV hits under his belt to come. Nevertheless, these stories do lend the experience of watching Incubus more unease than it might otherwise have had, especially as it is so concerned with the occult, and its faux-European art film atmosphere contributes to its sense of otherness. The fact that everyone in it is talking in a language that so few people understand helps towards that arcane mood as well, naturally.
As to the plot, Shatner plays Marc, a soldier who has returned to the island after years away at war - which war and even when this is supposed to be set are never explained - and he settles with his sister hoping that time away from the conflict will refresh him. He's already feeling better when Kia sets her sights on him as she wishes to destroy a soul which belongs to a good and decent man instead of the usual dissolute types she usually gets to deal with. Throw in an eclipse which blinds Marc's sister and the creature that Kia summons to wreak vengeance on Marc after she falls in love with him, and you have a plot which bears the whiff of misogyny, as the females are either punishers or punished, with the men victim to their whims. Otherwise, what you have is a very pretentious horror which has more power thanks to the events around it rather than the actual quality of the presentation, but even that has its compensations for your viewing intrigue. Music by Dominic Frontiere.