The Devil is a Part-Timer Novel Volume 21 is written by Satoshi Wagahara and illustrated by 029 (Oniku).

With the Devil King Castle closed and the war between the humans on Ente Isle resolved, all that remains for Maou and his friends is to mount an assault on heaven. But before that, there’s something he needs to do-get dressed up to visit Chiho’s father and do the right thing. As the final battle fast approaches, Emi can’t help but notice that the Devil King seems to be in poor health… What hidden truths await Maou and company in heaven? Can they really take down a god? And if they do, what will become of their lives in Japan?