>>34243223Here:
>https://archiveofourown.org/works/11420757/chapters/25586208It's actually Perfect Son.
It's a Lusamime x Sun, umm... rape, mental reprogramming & surgery, eldritch abomination posessing fanfic.
It's not that smut centric and is a really good read imho. Spoiler for the most recent chapter
Lillie gets posessed by Nihilego and rapes Sun at knifepoint.