[Elemental Swordsmanship That Isn't Just Special Effects]
After transmigrating to the Warring States period, his mission is to eliminate the Demon King Muzan?
Shangchuan Qingxuan glanced at the young man beside him, dressed in a scarlet haori, and thought that as long as he warned him to watch out for the scumbag boss's “Three Thousand in One Breath,” things should be fine, right? Such a simple mission, surely nothing could go wrong...
[System Notice: Your affinity with this world is too low. Your talent, “Elemental Affinity,” will make the world reject you.]
[System Notice: You have contracted a rare blood disease. Remaining life expectancy: 3 years.]
Shangchuan Qingxuan, who didn't have Muzan's fate but had gotten Muzan's disease, was stunned.
“Scumbag boss! Get your attending physician over here. It's urgent!”
One of the few Demon Slayer fanfics I could actually read was one where the protagonist's appearance seemed to have no impact on the plot whatsoever—meaning it made no difference whether the protagonist was there or not… except for Kanae not dying. The only downside was that the protagonist didn't even go to save Tanjiro's family.
The author resurrected me
Overall, it's very good, with a few minor flaws, but they don't affect the viewing experience. 4 stars.
It's really good, I like it a lot.