If it was about condensing True Illusions, Yan Qingying would need to consume a lot of energy, but if it was about spreading phantom shadows, that was just an innate ability, like breathing for humans. Although it wasn't without energy cost, the amount consumed was negligible.
After gaining the Shadow of Death ability, Xin Yun personally experienced that facing the continuous assault of thousands of Phantom Spiders was absolutely impossible to withstand by brute force. What was most unbearable was that no matter how many were shattered, they were all merely attacking illusions, and it was completely unclear where Yan Qingying's true body was.
Facing the overwhelming influx of Phantom Spiders, it was impossible to ignore. No one could bear the assault of thousands of Phantom Spiders. Although each phantom contained only a trace of Death Yin Force, the constant barrage from thousands was simply insurmountable, no matter how strong one was.