Young Man's Three-Foot Sword, the Best Here

Thousands of years ago, demons appeared everywhere, preying on humans, and wars raged. Immortals came to earth and joined the four great beasts: Qinglong, Suzaku, White Tiger, and Xuanwu to lead the human race to unite against the demons. This battle lasted for thousands of years... The immortal finally used the dangerous places in the human world such as North Kunlun, South Liyang, East Cuihai, and West Mingyuan as the base of the formation, with the immortal body as the center of the formation to set up the heaven-locking demon-sealing formation to suppress the sealed demons. The mythical beasts sit in each of the four formation bases and protect them. After the war, the human race established four major kingdoms: Beixuan, Nanming, Dongli, and Xiling to guard the human world, while the fish that slipped through the net among the demons continued to secretly plot to subvert the human world like rats in the dark and smelly ditch... Thousands of years later, the princess of Mingxuan Palace in Beixuan Chang'an City gave birth to a son named Su Linchen. Su Linchen was gifted with cultivation since he was a child, but the demons will come back again. Let's see how Su Linchen holds a three-foot green blade and kills all the demons for nine days. (Slow pace + traditional immortality + fantasy + genius male protagonist + single female protagonist + no system + no rebirth + kendo + having a knife + it will slowly become more fun)