Demon Slayer: As the Demon King, I Will Change Everything

(Demon King-level strength + System (low-key) + six new Upper Ranks + action-packed battles + making up for regrets + no tragic deaths + everyone survives + original plot + some plot changes) Han Xia is a devoted Demon Slayer fan, but the author, Gotouge, was a little too ruthless and killed off too many great characters: “Gotouge, how could you be so cruel? You killed off most of the key characters in a single arc! I can't accept this! If I could transmigrate there, I could definitely change that ending!” As it happens, fortune seems to favor him. By a miracle, he transmigrates to the world of Demon Slayer, and with the arrival of his golden finger (which plays a fairly minor role in this story), he gains power on par with Muzan Kibutsuji, becoming the second Demon King after Muzan! He inevitably goes through all kinds of ordeals, but chooses to enter into a public alliance with Muzan (the two have actually fought, but neither could defeat the other). Meanwhile, he secretly builds up his own forces, plotting for hundreds of years, all so he can wait for the start of the Demon Slayer story and join forces with the strongest Demon Slayer Corps of the era to wipe out Muzan! Muzan: “You! You actually betrayed me! You went over to the Demon Slayer Corps. Why? Wouldn't it be better to become the ultimate life-form? Why cooperate with those foolish humans?” Han Xia: “You disgusting bastard, lower than a dog, and you still have the nerve to bark like that? Get to hell already!” And so an exciting story begins... (A new author is just getting started. Please be kind if it's not to your taste, and bear with me.)