The big final other-dimension demon race bad guys in the Great Ruler are beaten by Mu Chen and the hole between dimensions is sealed. However the demons corrupted/mated with humans in order to have a some traitors/minions they could sneak in amongst the humans. They were part demon, part human Mu Chen killed a bunch of them but it is completely possible that the demons mentioned in this series are the escaped descendants of the mixed-breeds. I suppose that they could also be remnants of the original demons, but Mu Chen became uber-powerful god-tier at the end of The Great Ruler and swore to wipe the demons out completely. Thus if these are those same demons it would be kind of contrived. Another clue is the title, The Heaven's List, at the end of The Great Ruler, the super artifact which determines the fate of the world was machine translated to "sky ranking" which might also come out to "Heaven's List." One more clue, the final boss was a demon with ten magic/demonic eyes and the more he opened the more powerful he would be. The eyes seem to be a theme in this series. One other thing is that Mu Chen did not protect the demon hybrids, even the ones who did not want to fight the humans. The hybrids were raised in creepy human cattle farms and their ancestors were captured by the demons and the human powerhouses could not/did not help them. So you end up feeling pretty bad for them.