A variation of the Vampires idea-
A modern day neophyte witch has attempted to summon a demon to further her magical abilities accidentally summons a sort of vampiric version of an Incubus instead. The demon, who feeds on the life force of women through sex, quickly dominates and feeds off of her- escaping during the feeding-induced coma. When she wakes up she discovers that while she was in her coma he's fed off of at least a dozen other women, putting them into similar comas that seem to be getting longer as he gains strength. Taunting her over an established psychic link, she quickly has to make a choice regarding what she's going to do about the Incubus- and the only chance she has at regaining control could mean agreeing to be his breeding mare for eternity.