I'm Really Not an Undercover Agent Sent by Zhengdao.

Li Mie traveled through time and possessed the body of a hall leader of the Qingming Sect of the Demon Sect with the same name. It would be fine if he was just the master of the Demon Sect. However, based on his memory, Li Mie learned that he was still an undercover agent sent to the Demon Sect by Zhengdao. Damn, why are there so many young people in the world of cultivating immortals? Do you think this is playing Infernal Affairs? PS: This book is also known as "The Villain I Faced Can't Be So Stable" and "To us, he is the European Emperor in the world of immortality"