Natural markings + natural transparency + complete sun breathing = Muzan's new father

(Not a time travel story, the protagonist does not know the plot)

Chiba Yang, whose everything was taken away by the demons, joined the Demon Slayer Corps at the invitation of the Flame Pillar and became the stepson of the Flame Pillar. From then on, all the demons ushered in their nightmare.

The protagonist is a descendant of the Jigoku clan. He looks very similar to Yoriichi. He comprehended the Breathing of the Sun on his own and created his own sword skills.