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～

Synopsis

To fight the monstrous "Outsiders" invading from outer space, the world's strongest Hero and Demon Lord joined forces. However, their alliance was ultimately defeated. In a desperate bid to change history, the Demon Lord, with the help of Hero's secret technique to reverse time and undergo reincarnation. Reborn as a boy who was bullied and enslaved in noble society, the Demon Lord now faces a cruel new reality. "Why is a slave so strong?!" "Strong? He’s not just strong—he’s a monster!!" With his overwhelming strength from his previous life intact, the Demon Lord trains relentlessly to rise to the top and ensure victory this time!