Power of Blood

Su Zimo laid on the stone bed motionlessly and listened intently to the weird sound that was reverberating through his cave abode. Feeling the sensation of the flow in his marrow, a series of realizations came through to him like seeds germinating.

This was the reason why The Mystic Classic of the Twelve Demon Kings of the Great Wilderness was getting more difficult to cultivate - it was a process of internalizing external qualities.

The skin was the first to be tempered, followed by the flesh, tendons, bones. After that sequence, the marrow would be refined - that was the Marrow Cleansing section.

External cultivation techniques were relatively easier to master.

That was because apart from sutra, breathing and expiration methods as foundation, there were other killing techniques used to refine one's skin, flesh, tendons and bones.