Zhan Yanhui exposes the real culprit.

The next morning, the doctors had declared that there was nothing that could been done about Consort Xi's condition and they could only be prepared for the worst. The emperor finally decided to mete out punishment on the Noble Consort so he went to visit her palace along with his grandmother, the Grand Empress Dowager. The empress and the rest consorts also decided to follow him since they would get to witness the demise of their greatest enemy. 

Meanwhile, Zhan Yanhui remained composed and calm even as the emperor stared at her in silence. No one dared to say anything as they watched the scene in worry that the emperor would find a way to save his beloved noble consort. Indeed, they were not wrong as they heard the emperor say. "Zhan Yanhui, this is your final chance. Give me the cure to Consort Xi's poison. I would forget this ever happened if you repent."