Do you know this one ?

Hi, I've been searching for a bl that I dropped a long time ago. I just remembered it but can't find the name. The plot is about a world where humans can develop a shadow. But when they do, they are sent to a special prison because the shadows make them dangerous. We are following a boy who was wrongly sent to a prison because of his shitty friend who wanted to hide his own shadow. So he is in the prison and there he meets a man entirely made of shadow. The shadow can progress in the body and the man was here for a really long time. his name was number one, or first or something. I don't know if it's clear enough, do you get it ?