in this BL (i think) MC is a god/demon and he's going through worlds playing as the "villain" and in one of the arcs he creates either Frankenstein or Cthulhu and ends up accidentally taking care of them, and they end up becoming a yandere- a lot of the arcs follow this same pattern. i think this was one of the first novels i read on this website, and i didn't end up adding it to my read list lmao.
I believe it's the 2nd arc of this novel. Boyfriends Always Turned out to Be a Horror Movie Boss https://www.novelupdates.com/series/boyfriends-always-turned-out-to-be-a-horror-movie-boss/