Dainana Maouji Jilbagias no Maou Keikokuki第七魔王子ジルバギアスの魔王傾国記; Dai Nana Maouji Jirubagiasu no Maou Keikoku Ki; 7th Demon Prince Jilbagias, The Demon Kingdom Destroyer

Alexander, whose hometown was destroyed by the demon race, fought with the Demon King and was killed. But instead of the afterlife welcoming him, when he opened his eyes, he was reborn as the Demon King's son, Jilbagias!? While hiding his true identity, he stains his hands with atrocities as he conducts himself as the ideal demon prince, increasing his power to ultimately overthrow the Demon Kingdom...