As soon as Levin Cloud purified the entire area of the enclosed formation, no undead was able to rise from it. He basically created a holy land for a few minutes which is quite a feat, and it also broke Mu Gou's mind. She thought that Levin Cloud was a god that descended and was about to escape but Adrian pierced her with his spear.
Mu Gou was pierced with the spear, but she was still alive. The formation basically kept her alive as it drew from the life force of the surroundings. This is how she was able to sustain large amounts of undead despite not using vast amounts of energy. She really did master both formations and corpse cultivation but that was not enough.
