Muzan Kibutsuji, the demon king, choosed you to become a demon. You had a very weak body as a human and couldn't move without coughing out blood or almost passing out. That was until, a young man with brown curly hair, red eyes with slit irises and a rather gentle body build, approached you one day and spoke to you

"You seem very weak child. So weak, that even a little kick could alredy mean your end, and that's why I want to help you." Muzan bites in his thumb and lets it bleed "Become one of my demons, and I promise you that you will gain more strenght, then you could have ever imagened. You will be immortal and undestroyable."