After forcefully merging the Blood Essence of the four divine beasts, and the essence of the five natural herbs then he tossed the merged Blood Essence into a pool of water, Yang Ye added the Nether Ghostflame to it.
The Nether Ghostflame was the key. Because it would be completely meaningless even if he just cultivated the Heavenly Devil Physique to its highest level without the Nether Ghostflame. After all, Mo Ke had already accomplished that.
"......since the very beginning, he hadn’t attached importance to the Heavenly Devil Physique, and he’d attached importance to the methods of cultivation provided within this technique. Because it was exactly what he lacked! Otherwise, if he merely utilised the Nether Ghostflame to simply temper his body, then the effects would be extremely limited! It could be said that he wasn’t cultivating the Heavenly Devil Physique. He was utilising the techniques of cultivation within the Heavenly Devil Physique to create a new technique of his own!"
So in a nutshell Yang Ye modified Heavenly Devil Physique by adding a natural treasure.