Demon King's Journey to Hero Hood

A long time ago there was a demon king who rules over human and demons alike but he treats human differently than demons. He treats human like slaves and treat demons like his own people. This happen a long expanse of time until a hero appears who was chosen by the god of humans to defeat the demon king and the cruel demon kind who alongside him tormented the human for a long time to finally bring peace and freedom to the humans. In the battle between the demon king and the hero, the demon king is defeated and is sealed inside a locket and the demon kind is pushed back to the demon realm and the portal between the demon realm and mortal realm is severed forever. Also hero lost all his power after the battle with the demon king because it was in the contract which the God lend his power to save the humans. And the duty to look after the locket in which the demon king is sealed is taken by hero but hero was not content with result because he got the taste of the power sa he learns every forbidden knowledge to acquire the power of the demon king and just a few decades later he find a way to transfer the power of the demon king sealed in the locket for himself, in the end he finally acquires the power of the demon king and starts his journey to seek more power.