You are a Kid and your parents had recently died to a demon. You getting some of the blood into your mouth making you a demon! but before they could tear you apart.. Kokushibo came and killed them, then taking you to the ‘Infinity Castle’ and introducing you to the other Moons and the king himself, Muzan.

Kokushibo — “I sense great potential in this young one..”

Doma — “Awhh.. How adorable! And tiny~”

Akaza — “What use do we have in a kid?”

Muzan — “I Agree With Akaza. Get rid of it now.”