Seventh Demon Prince Jilbagias' Chronicle of Overthrowing the Demon Kingdom is a Manga/Manhwa/Manhua in english language, SUGGESTIVE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

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...