The Knights of Gangnaam is a Manga/Manhwa/Manhua in english language, manhwa 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

An unknown disaster strikes the splendid modern city of Gangnam. Souls to all monsters and demons Wizards appearing from the sky, land and sea at the same time. Gangnam is transformed into hell for enemies of a different dimension that cannot be matched with modern weapons or science.