Kimetsu No Yaiba: Rengoku Kyoujurou Gaiden summary:

Since ancient times, rumors have abounded of man-eating demons lurking in the woods. Because of this, the local townsfolk never venture outside at night. Legend has it that a demon slayer also roams the night, hunting down these bloodthirsty demons.Set before the events of the main series, this spinoff tells the story of Rengoku Kyoujurou, an energetic young demon hunter, as he takes over his family’s role as the Flame Pillar.