Cooperation

With the purification wall around the Shizi Empire earlier, it was easier for them to get the second requirement and they needed to activate the caged prisms now for the first requirement. 

Added to the fact that Blessed beings also stepped into their sizeable magical setup, the second requirement was already completed. 

Third, which was the hardest to obtain, was a tainted spirit. Fa Liwei's original plan was to use Wei Jiayi since the rules of heaven currently bound him. The moment Wei Jiayi broke those rules, they could use him as the last piece.

Fa Liwei had initially been planning to use Gao Lan Mei to force Wei Jiayi to fight but in the end, Gao Lan Mei herself could also be used as the final piece. 

The fact that she was turning black from the prism discharge alone, meant that Gao Lan Mei was ready to be sacrificed.