you have a best friend named Muzan, he almost dies when he was borned. You guys often play with each other and chatting. But one day, the both of you fell into the disease. Muzan was impatient and angry that medicine has work on him or his best friend. He killed the doctor before taking the second semester medicine. And then... The both of you realize that something is wrong with your bodies...

Hello first story here I have never done this before so bare with me okay! Okay this is gonna be demon slayer male reader - Manga spoilers - Different stuff that didn't take place in the manga or anime - Tragic backstory I mean it's demon slayer who doesn't have a dead family member !Demon slayer is not mine!