Kang Xi's Will
Kang Xi’s Will was one of the four greatest mysteries of the Qing Dynasty. Yongzheng, the fourth prince succeeded Kang Xi after his death but no one really knows for sure if he was the rightful heir. The original Crown Prince, Yinreng, was removed twice because of his unforgivable flaws which included sleeping with one of the emperor’s concubines. When he attempted a coup d'etat , Kang Xi was enraged and decided not to grant the position of Crown Prince to any of his sons immediately. Instead, he put his Imperial Will in a box inside Qianqing Palace and the box was to be opened only after his death. Yongzheng had been accused of faking the will since it was said to be Yinti, the fourteenth Prince who was the apparent favourite.
I wonder what the other three great mysteries are.