The First Demon king, The Hero's Rebellion!

A year has passed since 40 heroes were brought to this world to purge the Evil from it, A year has passed since I destroyed the the Demon lord who amassed his armies and nearly brought the world to heel. A year has passed, my boredom has risen, I was considered the strongest of the Heros. A magic warrior, a martial artist fuelled by mana, able to destroy both the soul and body of my enemies. Married off to some wife in the south of the kingdom, to be forgotten and buried along with the other male Heros. I control the small dukedom between the Azure and Demon Kingdom, so imagine my surprise when I get a quest; 'Unite the demon clans, rule the world, destroy humanity, become the first Demon King.' Finally, my boredom can end....