Countess Barbara (Claudia Gravy) returns home after a year of gallivanting around Europe, much to the delight of lovestruck Duke Lionello Shandwell (Mark Damon, who looks like a cross between Doug McClure and Martin Landau). A couple of problems: 1) she announces that she has got married, and 2) Lionello is her brother (Barbara is a total babe, but that's no excuse). Jealous Lionello proceeds to spy on his sister and her new husband Giordano (Aldo Bufi Landi), including when the couple are making love; meanwhile, a mysterious figure is killing local women with a three bladed weapon. Giordano comes to suspect that Lionello is possessed by the demon Byleth, who causes its victims to kill and live in incest.
Byleth: The Demon of Incest features plenty of nudity and sex, with some very attractive actresses revealing all for their art, but despite the film's salacious title, it really isn't all that depraved. Shame. With a little more deviancy and a lot more gore (the murders are extremely tame, with just a few trickles of blood), this could have been a classic of Italian sleaze, but as it stand. it's actually surprisingly bland, nothing much of interest happening between the soft-core romps and lacklustre deaths.
The incest angle finally comes into fruition moments before the end, when Lionello gives in to his urges and Barbara willingly obliges (he's possessed by a demon... what's her excuse?), but the sex is only suggested (keeping it 'tasteful'). Lionello wakes to discover Barbara dead, three wounds on her neck, and goes in search of Byleth for revenge, leading to a rather confusing climax in which the demon appears, looking like Lionello in drag-queen make-up. As erotic gothic Euro-horrors go, this one is merely run-of-the-mill, not nearly as controversial or taboo-busting as the title suggests.
