Undercover for Ten Years, I Became a Great Villain of the Demon Sect is a Manga/Manhwa/Manhua in english language, Manhua 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

He was sent by a right righteous to be an undercover cultivator in the Ten Thousand Demons sect for ten years, but was accidentally pushed to the position of sect master! Since then, Luo Yunxiao started a different path as the great demon!