In the process of merging the middle dantian, the nine great immortal meridians, the Nascent Soul, and the body, the most difficult hurdle was the pain and rhythm.
After all, doing so was to separate the middle dantian, the nine great immortal meridians, and the Nascent Soul bit by bit and fuse them into various parts of the body. The degree of pain was ten times or a hundred times more than being cut into pieces.
The speed of the fusion could not be too fast or too slow. If it was too fast, it would cause the body to collapse. If it was too slow, it would have to endure an even longer period of pain. Once it could not endure it, it would cause a mental breakdown and the Nascent Soul to lose control and die.
