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

The great orthodox-unorthodox war that divided the world of martial artists ended with the victory of the demonic cult. Despite the domination of the demonic cult, the successor of the demonic cult was assassinated as a part of a great scheme, and the brother of the successor returned from the shadows to get revenge. The eldest son of the demonic cult, Hyuk Ryeonhwi, is a survivor of the Purple Cloud Island, a place also known as the land of the dead. He infiltrates the top training facility in the demonic cult in order to get to the bottom of the scheme and ends up meeting a secret master martial artist of the orthodox faction, Bi Seol. The two help each other out and develop special feelings in the process… Thus begins the domination of Hyuk Ryeonhwi, who will eventually come to be called the demon king.