A Contract Of Adversity

This is the tale of a young boy who one day came into contract with last Demon King to become the next Demon King. Since that day he's been into many battle and war as of his father who current Duke, but to escape that he finally tries and gets success in escaping to the world which his grandpa visited as kid, where was no war at all. But the despair remains as he finds that there is also war going on and he starts helping people who are having trouble during war while fighting and making friends with different militaries and even has to deal with supernatural creature from his own world in the modern world as well.