Maou to Yuusha no Kakeochi Bakansu

A hundred years. Humanity and the Demonic Race loathed one another, locked in a war. When the fated time arrives, the Hero and the Demon King sought each other out with the intention to kill. Yet, 5 years later... The two are in the middle of their lovey-dovey vacation on a southern island!? How did this happen!? Where does their romantic getaway lead to...?