Aside from wanting to take out both Yunxuan Villa and Lu Xuan, the main reason why those people chose to unleash the demons in Fu City and around the villa was to act against the Region of Huaxia.
When the seemingly endless horde of demons was unleashed, the forces of the Region of Huaxia would have at least been cut in half by the incident; it didn't matter how powerful the region was back then. Besides, it was also very normal to see deaths amounting to over 100 million.
The number of casualties incurred among the formidable cultivators would have most likely been so astronomical that it was difficult for anyone to fully comprehend for themselves.
By then, the Regions of America and Europe would have become the most powerful regions in the Human Alliance. Thus, they would have gained the power to truly steer the direction of the Human Alliance.
