Not only that, the hell of reincarnation in the eight desolated universe also produced many more souls of demons. Some of the souls actually turned back into human form after passing through the gate of hell and the river of forgetfulness. They were reincarnated after drinking the soup of granny Meng.
These were primordial Immortals who had turned into fiends. They were the gods and devils of this universe. They could not escape from this universe, so they chose to fall into The Fiend form and seek another way to be reborn when the universe was destroyed.
In fact, these primordial fiends were excellent materials, especially their souls. Unlike immortal gold Immortals, they were indestructible, and could not be destroyed so easily. They were excellent materials for refining primordial immortal treasures.