Dungeon Builder: The Demon King's Labyrinth is a Modern City! (Manga) Vol. 6

After obtaining a forbidden power, Procel learns of a senior demon lord's plot against him. Unwilling to let another lord belittle his townspeople or force them to submit to outside rule, Procel plans a counterstrike against the oppressive neighboring town, Eklava. But as fear and despair grow, so too does Procel's untapped power as a demon lord. When he learns that his fellow demon lord, Marco, is facing an attack, he must put that new strength to use, beating back the oppressive force!