New Dawn: Demon Tanjiro's Awakening

In the aftermath of the climactic battle against Muzan Kibutsuji, the Demon Slayer Corps mourns their fallen hero, Tanjiro Kamado, who has tragically transformed into a powerful demon. Despite his friends' desperate attempts to save him, Tanjiro succumbs to the demonic influence, losing his humanity and becoming an even stronger threat than Muzan himself.

(it will be large fanfic. The story begins after chapter 200 from the Manga)