That whole demon qi is probably the worst detail in this whole series. Even emperor level cultivators simply get demonised by breathing in a bit of demon qi. And then those demonised cultivators immediately switch sides and cooperate with each other. How does that many any sense? It would make barely a tiny bit of sense if they just turned berserk, unable to resist any impulses, attacking anyone nearby, including other demonised people naturally. But now just breathing in a bit of demon qi suddenly turns them into soldiers of an enemy race. Why doesn't monster qi them turn humans into monsters? Why doesn't the various human qi turn demons into humans?
The author can't explain any of this ridiculous nonsense, so he doesn't even bother to try.
Why isn't Yang Kai using his insects against the demon spirits and demonised cultivators? He last time did it in the mirror world and it was exceptionally effective. Now the author has forgotten Yang Kai has those insects, just like he forgot for an exntended period that the golden embodiment exists. Perhaps the author is sleep-deprived and it shows.