Demon Slayer: Building a Faction in the Demon Slayer World

Lin Yu is a modern man who, after finishing Demon Slayer, unexpectedly transmigrates to the world of Demon Slayer. He happens to transmigrate right onto Muzan's face, and Muzan turns him into a demon. Only after he becomes a demon does the System finally show up, allowing him to escape via teleportation. With the System's help, he suppresses some of his demonic traits, breaks free of Muzan's control, and becomes half-human, half-demon. To make it up to its Host, the System rewards him with the method for using Breath of Light, then falls asleep from overexertion. Knowing the story, he decides to build his own forces and set off on the adventures to come... (A dark-leaning story. Don't read if that's not your thing.)