The Reformation of the World as Overseen by a Realist Demon King, Vol. 5 (manga)

Astaroth has been reincarnated into a strange new land as the seventy-second Demon King with a mission from a mysterious goddess to unite the world under his banner and reform its chaotic ways. With two demon lords now crushed beneath his heel, Astaroth's power is only increasing, but he's got no time to rest on his laurels. In search of a means to bolster his army even further, he heads to an abandoned ruin, where he comes across a mysterious boy...