The zombies were a product of an ancient Saint's pursuit of eternal life. At the time, he was on the verge of death. Unable to enter the God Realm to increase his lifespan, he had created a certain technique called the Zombie Devil Technique, which required him to first die to gain eternal life. 

Thus, he buried himself alive to cultivate the technique. After 10,000 years, he emerged from the ground as a zombie whose thoughts had been preserved. With his technique, he also created numerous zombies, eventually forming the zombie devil race. 

A regular corpse would undergo numerous transformations before becoming a walking corpse. Only at the final phase of the transformation would the walking corpse obtain the zombie...