Of course, most of the so-called "wildlings" were the humans who escaped to the underground world in the Great Dark Age when they could not withstand the tyranny of the demons anymore as well as the demons who escaped here in exactly the same way when the reign of demons collapsed.
That was because the environment on the border of the crust and the mantle was terribly harsh. Not only was the breathable oxygen lethal here, but the temperature was also ridiculously high. Normal human beings could not get used to the wilderness that did not have an air circulation system or cooling rune arrays, and they would be gone soon after they entered the rough country.
Demons, as well as the humans governed by demons in the "Great Dark Age", however, boasted their unique advantages.