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

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!