I am not an English expert (as you requested one), but I would think that "spell" means something like "fascination" in this case, like in the idiom "being under one's spell" https://idioms.thefreedictionary.com/under+her+spell
The synopsis for Silvia's Spell 1: "A beautiful princess and her lover, the prince, are helpless under magic spell cast upon them by an evil, jealous witch."
Silvia's Spell 2: "The year is 1892 and after centuries of hiding in shame as a condemned sex object, a young princess (Silvia Saint) is forced to serve an evil witch in a whore house. Will her prince be able to find her and free her from this evil curse?"
Silvia's Spell 3:
"In the final chapter of this anal extravaganza, Silvia Saint stars as a horny princess trapped under an evil spell. Can the Prince rescue her from her sexual slavery, or will the Evil Witch continue to make her **** at will?"
So "spell" actually means a magical incantation or curse.
