Man, I see people get tripped up by this all the time in the fan circles. The 'Demon Butterfly Story' most folks are buzzing about is actually the Chinese web novel 'Demon Butterfly: The Corpse Collector'. The protagonist is a dude named Lin Feng. He starts off as this modern forensic student who gets yanked into a bizarre ancient-fantasy world where he's suddenly a lowly coroner's apprentice. The whole hook is that he uses his modern forensic knowledge to solve supernatural murders in a cultivation society that's totally baffled by concepts like fingerprints or blood spatter analysis. It's a wild mashup.
Lin Feng isn't your typical overpowered isekai hero swaggering in from day one. He's clever but physically weak in this new world, and a lot of the tension comes from him trying to stay alive and prove his worth while navigating court politics and cultivator arrogance with just his wits and a scalpel. Some readers find his progression a bit slow, but I think that groundedness is what makes the 'butterfly' part of the title work—his actions, like a butterfly's wings, set off huge, unforeseen consequences later. He's more of a detective in a xianxia robe than a martial artist.
Honestly, the title threw me for a loop at first too. I expected some ethereal beauty or a tragedy, not a procedural with spirit parasites. But Lin Feng grows on you. He’s pragmatic to a fault, which leads to some darkly funny moments when he's calmly examining a corpse while everyone else is vomiting or chanting spells.