異世界魔王の奴隷譚～転生したら奴隷だったので、仕方なしに瞬時に成り上がろうと思います～

To counter the monsters from outer space known as "Outsiders," the world's strongest Hero and the Demon King joined forces. However, the world alliance led by the Demon King and the Hero is defeated. Thus, through the Hero's secret technique, the Demon King travels back in time and undergoes a Retrograde Reincarnation to change history. The destination of the Demon King's reincarnation was a boy who suffered from brutal bullying in aristocratic society and fell into slavery. "W-Why is a slave so strong!?" "It's not just strength!! He's a monster!!!" Keeping his overwhelming strength from before his reincarnation, the Demon King trains himself to win this time and rises again!