Cosmic Gaything

I could have liked this if 1) there was a fleshed out plot that was focused on something other than torture (like maybe the "girl" is actually an eldritch monster who feeds on pain), 2) there was only one couple, and 3) the guy was slightly into it. For example, their relationship could start out normal, with the girl slowly introducing BDSM elements that the guy gets used to. Then the girl reveals that she used to kill small animals or something when she was a child and has the urge to kill people as well, and only inflicting pain on her lover helps suppress that urge. So our guy decides to take that upon himself because he doesn't want to see his girl become a murderer, and it turns out he's somewhat of a masochist (but not enough to want to outright die by her hand).

Another idea is some kind of death game where there's a pain or injury quota that needs to be met, or the more injured you get the stronger you get. And so guy enlists his girl best friend to hurt him because he finds it too difficult to hurt himself. She would initially be horrified but they would both grow to experience a twisted enjoyment of the practice. There would also definitely be a character who compulsively SHs and is able to rapidly grow in terms of strength and abilities because of that but is also unstable mentally

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