The Reformation of the World as Overseen by a Realist Demon King, Vol. 6 (manga) — Ryosuke Hata

Now that Astaroth has already taken down the Demon Kings Sabnac and Eligos with the help of Eve, Hijikata Toshizou, and Jeanne d'Arc, he's set his sights on the star-shaped Demon King Decarabia. But even though Astaroth's plan to turn Decarabia's greed against him worked beautifully, the rival Demon King has a trick up his own sleeve. And when Decarabia's castle does fall into Astaroth's hands, he'll need someone capable to run it…perhaps even one of the greatest strategists of all time!