Yin Tulong's Divine Fire Nine Refinements was created by comprehending the Nine Dragon Divine Fire Cover, and when perfected, the nine dragons emerge simultaneously. Especially with Yin Tulong adding the Dragon and Snake Soul, it enhances its power even more.
Currently, the demons, ghosts, and monsters are essentially human, and when they die, the Dao Fruit emerges, with the vast majority of corpses reverting to human form. A few that cannot return to human form are due to either being unable to withstand the drastic changes or due to practicing some crooked cultivation technique, and are not truly demons.
In such a case, Yin Tulong wanted to mimic a Third Prince's actions of skinning and tendon extraction, but it was undoubtedly impossible. So, he turned to extracting the souls imbued with the Dragon and Snake Nature.
Skinning and tendon extraction seem no different from soul extraction.
