Okay, I have to say y'all's guesses are wrong because I (not so) accidentally spoiled myself by reading comments on the Japanese version and if they are closely following the OG script then the culprit would be the same.
Would anyone be kind enough to help me understand the Jingzhu Massacre from 17 years ago? So far, what I understand is that the late emperor was the root cause of everything. First of all, he wanted to get rid of his son, the Grand Prince at that time. But how did Wei Yin start his affair with that consort, and how did Qimin get swapped with Sui Yunhui? And how in the world was it Wei Yin’s fault for the massacre of 100,000 soldiers when it was Lord Chengxi who refused to send troops? And when the emperor was talking to Lord Chengxhi, General He was also there and was aware of the massacre that was going to happen, so why didn’t he do or say anything? And why was Fan Changyu’s father blamed for everything and labeled a traitor? Also, at one point it was mentioned that wei yin raped a maid when he was drunk so the emperor gave that maid to him. What was it about?
I feel like the 40 episodes restriction really sacrificed the story a bit. Had it been 50 episodes long, I'm sure they wouldn't have to explain everything so hastily and rush the last couple episodes.