The Demon King and the Hero's Elopement Vacation

For a hundred years, humans and demons have hated each other and fought without end. At last, the Demon King and the Hero come face to face, each intent on killing the other. But five years later the two are enjoying a romantic getaway on a tropical island!? What happened!? Where will their runaway romance lead? (Source: Kadokawa) Note: Includes one extra chapter.