The God Slaying Demon King is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

The strongest demon king in history, Hairam, dominated the continent of demons with overwhelming strength. However, after defying the Demon God’s oracle, which demanded sacrifices, a fierce battle commenced that ended with Hairam’s death. “The demon race cannot possibly win against the one it is ruled by”, facing this harsh reality, even the strongest could not do anything. After 500 years passed, Hairam has reincarnated as a “human”. Although the human race has a slow growth, it has the highest latent potential among all races. Thus begins the journey of Hairam, who has inherited the powers of his previous life, to kill the demon god once again.