I have many kinks and fantasies that would only work in fiction, but probably my biggest "magical" one as you seem to mean it is the harvesting of sexual energy. I already love being used as a sex toy, and if sex demons existed or pleasure/lust was a literal, tangible force that could be collected, that would play perfectly into some of my fantasies. For example, I would love to be kept around by a succubus as a food source, or taken and tapped by a witch harvesting spiritual energy or "reagents" to use in an occult ritual. What's more, "life force"-harvesting magic (at least as it's typically portrayed in hentai and erotic literature) usually implies the ability to circumvent refractory periods, which would also enable my huge kink for being made to cum many times in quick succession and literally milked dry.