The size of a race's territory is directly related to the extent of its power.
To put it simply, the bigger the fist, the larger the territory. The Yaoling Mountain Range was divided among five great Demon Kings, and the Wolf Race lived under the protection of the White Demon King.
Different races pledged allegiance to different Demon Kings.
The Gray Demon King didn't have many races pledged to his territory, but the numbers of his own Rat Lineage were endless. Among mammals, rats had the absolute greatest reproductive ability. Their numbers were simply too great. It was said that if all the offspring from a single pair of rats survived for a year, there would be as many as five thousand.
Rats were impossible to exterminate, and their reproduction was even more exaggerated in a place like the Yaoling Mountain Range, which was overflowing with Spiritual Qi.
