Demon Slayer: Reincarnated as Muzan's Brother, the System Helps Me Kill Him

He transmigrates as Muzan Kibutsuji's twin younger brother, destined to die from the start. After more than a decade bedridden and on the verge of death, a dose of medicine transforms him into the original demon. His System awakens and he gains Blood Demon Art, but before he can celebrate, he runs into the newly turned Muzan and gets an entire arm torn off right in front of him, then tossed into the dirt. His parents take their own lives, his clan despises him, and his entire family curses him. Even the mountain girl who kept him company for a century can only grow old and pass away before his eyes, leaving behind a single sun-shaped earring. Through the long years, he hunts his own kind, takes in Tamayo, and glimpses the truth of the world. Everything he does is for the day he can catch up to the man who has long stood at the pinnacle of the demons. Now he wields a Nichirin Blade and shares the same original blood, so neither can use bloodline suppression against the other. The world believes sunlight is the only thing capable of killing the Demon King, but it is precisely the one taboo he cannot touch. If no one else can do it, he'll break this deadlock with his own hands. The hatred he has spent a century building is ready to sink into Muzan Kibutsuji's flesh. This novel and its characters are entirely fictional. Any resemblance to actual people or events is purely coincidental. Do not imitate.