This one has been a right pain to get right.   The whole thing started because I messed up a spell and got a permanent shock shader effect stuck on my character. Which was mildy annoying, but also looked undoubtedly badass. So, obviously, I wanted to use it for something. So I got the idea that if I had sex with anyone while the effect was running, it would spread to any sexual partners, and while the effect might be painless for the caster, it would be anything but for the victim.