THE ANOTHER WORLD DEMON-KING’S SUCCESSOR 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

Asato Kanoumi, who was summoned as a hero to another world Istalia. To stop the war between the demons and humans
It is said that he was summoned by Anna, the saint of the Kingdom of Elshnon.
After a year of war, Asato finally challenges the Demon King Lione.
Just when he thought the battle was over, his summoner Anna attacked him and the Demon King. Asato who survived the attack and was protected by the Demon King.
What is the future path for the hero betrayed by humans...!?