Luckily William planted tons of formations all over the underground city and portal. So, when the third wave arrived, it was literally impossible to keep them in check at the underground city alone. And the leading masters had to use the formations surrounding the portal area.
This was quite risky, as William stressed upon not using these formations until it was desperate. And it indeed was!
If they didn't do that, the coming masters from the outer world would have taken control over the underground city this way. So, Fang led the elite master group, went all the way down to the portal area using special tunnels they dug before, and started a massacre there.
Doing it at the portal area wasn't as bad as it was at the underground city surface. Masters coming out from the portal were exhausted and not accustomed to the pressure and decline of power they suffered when coming here.
