When remnant was created, the god of light and darkness had a fight, it almost destroyed remnant but they decided to have a truce.

They had created Angel and Demons. The Angels stayed in heaven while the Demons stayed in hell.

However, One demon had escaped hell and ate and turned a human into a demon. The gods were shocked that a demon had been able to escaped and turn a human into a demon.

They decided that human with a good heart who turned into a demon wont be burnt by the sun or need to eat humans.

Thousands of years later and demons and grimm were common knowledge.

While anyone can be a huntsman. Only few can become a demon slayer.

This is the story of tanjirou. The demon slayer with a sister who's a good demon.......Huh.

Wonder how that will work out.