Okay: "Headcanons for horror villains who are obsessed with The Reader, who already has an S/O?" This will have both male and female villains; Four characters, but you're free to add more. As for who to use: I have suggestions (Ghostface/Stu and Billy, Lily, Freddy Krueger, Pinhead), add whoever you want, like...Like a monster roulette, of sorts. Have the Reader be as reluctant as possible.
My answers were getting too long so I didn’t pick a monster for myself lol. It’s a toss up between the Babadook and the entity from It Follows.
Ghostface: Billy and Stu would try to get you to secretly cheat on your SO. Who doesn’t like thr**somes, right? They slowly become more and more frustrated that you’re so loyal you don’t even like being flirted with. Threatening you is the next option and Ghostface is perfect for that. And your SO is the perfect scapegoat.
Lily: It wouldn’t be long until she realizes seducing you away from your SO isn’t going to work… while your SO is around. As soon as Lily came to that realization, she’d kidnap you. Once you arrive at your love nest, she’d f*ck you into submission.
Freddy Krueger: Too bad invading dreams is his whole modus operandi. He’ll use your k*nks against you. And of course he has no qualms about slashing your SO.
Pinhead: Ugh, I should have paid more attention to his characterization. I have this scenario in my head where the reader is absentmindedly playing with themself while their SO is fiddling with the puzzle box. Pinhead is immediately intrigued but of course you want nothing to do with him or his k*nky… club. You’re desperate to save your boo, so you try haggling someone else’s soul. Pinhead allows it because he thinks you’re bodacious. Then there’s probably a lot of awkward one-sided flirting.