in this alternate universe,there is a myth about beings that are like demons but are pure hearted.Their leader changes to the next generation.They can't die by demon slayer or demon hands.They have rarely been seen by anyone. Their leaders like Muzan can turn humans into demons but they can turn moon demons to their side.Their is a curse on any demon slayer or demon slayer decendant that goes 'The first breathing technique was made not by a human but a demon.Once a human met a sun demon leader.The human asked for a way to stop the evil demons so they leader said that sun breathing can defeat a demon.The leader warned that the sun breathing and the ones after would have a curse.The curse it that the demon slayers would have sun demon blood in their bodies to help defeat the demons.The human understood that there is always a price to pay for something of use to anyone.The man learned it and had demon blood in his body that was mixed with human blood.He defeated demons and later taught others it.' The sun demon the first ever demon to have ever been made.They could go into the moon's and sun's lights they were the only demons who could do it though.Few people have ever seen a sun demon. Sun demons don't hurt humans they hunt dark and evil demons that muzan made.Nezuko was turned into a moon demon but her dad was a demon slayer so sh had sun demon blood in her but it was only a little amount of it though. Y/n like Nezuko was turned into a demon but was found by the demon leader who took her in and turned her to the side of good.She trained with the sun demons then at the same time as Tanjiro joined the demon slayers.