Why I Quit Being the 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.cc fastest updating comic site. The Summary is

[Chapters will be added once out]
In a time of chaos, the Demon King traversed through 665 realms before invading the human realm. It was a brave hero who ceased the war. Thanks to their divine sacrifice, mankind barely managed to survive. But that led to the story of the new Demon King. Meet Demon King Demiourgos, the 666th. “I quit being the Demon King,” he said, before leaving the demon realm. This is a story about a runaway Demon King and a hero who becomes his subordinate. Why did the Demon King quit?