Three types of Wandering Corpses, Three Corpse Transformations, no matter how they change, cannot escape this realm. Only when reaching the Undying Bone, does the true transformation begin.
The Undying Bone is when a bone is refined to the extreme. When spiritual essence is gradually infused into it over time, and a certain bone in the body gains spirituality, it is considered entry level.
So when is the Undying Bone considered complete?
When the bone is cultivated to not only contain spirituality but also requires the constant deepening of the body's death aura with the use of ultimate yin objects, until the cultivated bone transforms from death to life, only then is the Undying Bone complete.
In Taoism, there are three treasures: essence, energy, and spirit. To a certain extent, the Undying Bone is also the place where the three treasures of a zombie gather.