Having been away from the family estate and his beloved sister Barbara for a year, handsome young nobleman Lionello (Mark Damon) returns to a shock: Barbara (Claudia Gravy) has married the older Giordano (Aldo Bufi Landi) in his absence.
The issue is more than one of brotherly concern; in any case Giordano is a suitable husband in every respect, cultured, considerate and embodying all the virtues of his class. Rather, it is that Lionello, whose nerves have always been somewhat fragile in any case, has incestuous longings towards his sister.
It may be merely a coincidence, but Lionello's return also coincides with a series of murders, beginning with a prostitute, Dolores, in town and continuing with one of the family's own servants, the maid Gisella. In both cases the killer's modus operandi is the same, the victim having been killed by a precise strike to the neck from a distinctive three bladed weapon.
Coupled with a number of other signs – the sight of a white horse, a tremor – it leads the local priest to suspect that a demon, Byleth, may be abroad on the earth and in their midst.
Lionello the voyeur #1
Meanwhile, Barbara and Giordano have invited the beautiful Floriana (Silvana Pomilli) to come stay with them, in the hope that a romance with Lionello might blossom or, if not, that her presence will at least keep him from brooding on things...
Lionello the voyeur #2
Written and directed by the enigmatic Leopoldo Savona, Byleth – il demone dell'incesto is one of the more ususual possession-themed films to come out of Italy in the 1970s on account of its historical rather than contemporary setting and the nature of its victim. Both these distinctions can presumably be put down not only to the film's pre-Exorcist origins but also its somewhat more aspirational nature, whereby the sex side of the sex and horror material is often framed in the context of Lionello's sickness and voyeurism in a way that makes us aware of our own 'sinfulness'.
Lionello the voyeur #3
The evocation of period detail is effective, with some nice details like the investigator's suspicion that the prostitute's murderer may be one of the Carbonari or Barbara's discussion of the thrill of travelling on a fast-moving train and of visiting Venice and Rome.
Thought at times reminiscent of Bava's Kill Baby Kill, with Savona also using zooms and close-ups to underscore key moments, the film lacks the same kind of clash between modern and traditional beliefs, with the idea of demonic possession being accepted more matter of factly after it is raised – a distinction that can be attributed to the film's being set 70 or 80 years earlier, at a point in history where the connections between madness and religion were that bit stronger, even among the educated elite.
Although Byleth also lacks Kill Baby Kill's more obviously expressive use of colour, its own colour palette seems somewhat worked-through, with natural shades predominanting to cloak many scenes in a sepia tone equally suggestive of hazy nostalgia of Barbara and Lionello's childhood games and the suffocating weight of the same past in the present where they are now forbidden, taboo.
From innocent play to something more serious in two minutes – Lionello and Barbara
Another obvious point of comparison is Joe D'Amato's undeniably muddled but haunting Death Smiles on a Murderer, with which the film shares its incest theme and that general air of decadence and malaise.
Giallo or gothic imagery?
Though perhaps inconsistently directed overall – a set-piece friendly duel turned competitive between Lionello and Giordano is full of dynamic camera movements and cutting, whereas expository scenes tend to remain bland and by the numbers – and lacking in a strong detective subplot or central character to engage the less committed viewer, Byleth works nicely when taken as a mood piece in which atmosphere is prioritised over shock and suspense.