A killer has been operating in a big city. The authorities have determined that it's a succubus, but they can't identify, catch and contain her. Strangely this succubus is targeting the wealthy rather than seeking to corrupt the pure, and she only seems to strike at lavish, exclusive parties. In addition to fearing for their lives, the upper crust are increasingly annoyed and embarrassed by what the authorities are sniffing up at the crime scenes, the implications of what they're up to behind closed doors. (Lots and lots of adultery, certainly, most likely some kink, probably ought to avoid heavy fetish stuff if you don't want a graphic and gross game but the duke that likes being tied up and spanked by hefty orc or hobgoblin women in serving girl costumes probably doesn't want his wife and kids to find out.)
The PCs are hired by some wealthy patron or another to identify and apprehend or destroy the killer. There's a substantial bonus if they can determine the reason for the unusual behavior, and who she's working for (possibly no-one but the wealthy tend to be paranoid about rivals). This succubus has taken appropriate precautions (items and/or spells depending on how much treasure you want to load her with and how much you want to power her up with class levels) to foil divination that would reveal her. The PCs will need to get themselves invited to, or otherwise successfully infiltrate these parties, mingle and set themselves as bait to draw the succubus out.
You'll have sexual themes all over the place, politics, mystery and investigation, the promise of shiny shiny gold, and maybe even an entertaining fish-out-of-water story as the barbarian tries to woo the high society women not with wit and charm, but his abundant physical prowess and animal magnetism.