Seduced by the Demon King

"I WILL PROTECT YOU NO MATTER WHAT." The Demon King swore to destroy his former rival by granting her true happiness in order to make her fall from grace all the more destructive. However, he failed to consider that Haruka needed him to fall for her just as hard in order to reach those blissful peaks. Now, the two have discovered absolute pleasure in one another, and life seems very nearly perfect. That is until Vanitas, the Demon King's loyal servant, launches a merciless attack to deal with an unprecedented crisis! Is the love between a former Saint and the ultimate Demon King destined for devastation?