When the Duke Lionello (Mark Damon) and his sister Barbara (Claudia Gravy, who appeared in some Jess Franco pictures but, as far as I know, never in any adaptations of the works of Robert Browning) were growing up on their family’s ancestral Lazio pile, they were such loving siblings that they play-acted getting married when they were older. Ah, cute. Barbara, as you would expect, grew out of this whimsical little fantasy… Lionello never quite managed to do so. When Barbara returns from a spell in Venice, her brother is overjoyed but she harshes Lionello’s mellow big time by announcing that she’s now hitched to Giordano (Aldo Bufo Landi). A big girl’s blouse in a frilly shirt, Lionello goes into angsting overdrive, moping around his castle, spying on the bonking couples with which it seems to be littered and enjoying his own odd assignations with prostitutes (very odd… he can’t seem to rise to the occasion with any woman who isn’t Barbara). He even hides in Barbara’s wardrobe, caressing her petticoats while he watches her and Giordano gittin’ it on through the keyhole,
Unfortunately a series of the women Lionello spies on and / or fails to satisfy start turning up dead, somebody having stabbed them in their throats with a three pronged knife. But who is that somebody? A handy dandy priest (Antonio Anelli) turns up to advise the police that such a weapon is traditionally handled by Byleth, the Demon of Incest, throwing in bonus biographical information about Byleth’s demonic cohorts , Astorath, Baphomet, Belphegor and so on…
In the rare moments that the screen isn’t filled with copulating couples, director Leopoldo Savona (better known for such endearingly titled Spaghetti Westerns as God Will Forgive My Pistol, Apocalypse Joe and Pistol Packin’ Preacher… also as the original director of what emerged as Mario Bava’s The Vikings knock-off, Knives Of The Avenger) and one shot co-writer Norbert Blake (anyone smell a pseudonym?) attempt to mix giallo elements into an already overcrowded supernatural-gothic-costume-melodrama-romance mish-mash and fail to pull it off because apart from the obvious suspect, no plausible red herring is even offered. Barbara finally (and a tad arbitrarily) succunbs to Lionello’s advances. We don’t actually see her doing so or him killing her, but it seems both of these things happened, ushering in a misfiring demonic wrap up.
Of the two films that most readily occur to me, right off the top of my head, as comparators, I like this one a whole lot better than Alfredo Rizzo’s The Bloodsucker Leads The Dance (1975) but it’s not a patch on Joe D’Amato’s Death Smiles On A Murderer (1973). Byleth is a rather minor effort, but the spaghetti exploitation cognoscenti will want to check out this interesting rarity from 1972. Severin’s 2K restoration has been sourced from an uncut (but somewhat damaged) German negative (as “Trio Der Lust”) with optional German or Italian sound and English subs. No extras.