Rimuru defeats Kleiman and officially becomes the Demon King. Moreover, after the feast of the Demon King, his territory expanded to the entire area of the Great Forest of Gula. Anticipating the arrival of representatives of various races, Rimuru decided to take the opportunity to hold the "Founding Celebration" to make his official appearance as the Demon King and attract new residents to Turnpest at the same time. At the same time, in Lubelios, the headquarters of the Luminus Cult that is hostile to monsters, the head of the Paladin, Hinata, receives a message from Rimuru. However, this message has been manipulated into a declaration of war. Rimuru received a report that Hinata was heading towards Turnpest, and what decision he would make. In order to recognize the true enemy and the true partner, and realize the common prosperity of humans and demons, Rimuru is about to take on a new challenge.