Helppp

I was reading recently about a two siblings being trapped in a cage along with some other kids. A boy and a girl. Turns out they were going to get sacrifices to a demon. There were important people that asked what was going on and the little boy said he’d tell them if they let him and his little sister out. Anyways theres this demon that comes later and some guy kills it within the stomach and takes both siblings in as his disciples. PLEASE HELP ME FIND IT! THANK YOU IN ADVANCE! ┗( T﹏T )┛