As was stated in earlier chapters, aideen killed the second prince on orders from the emperor and was supposed to return however the emperor sold her and her team out, fingering them as rebels trying to start a war or something in order to place his daughter next to the first prince, and had them executed while he had his daughter marry the first prince and probably, usurp the throne from him (mysterious poisoning or something is my guess, was never mentioned what happened after aideen's death), remember aideen's ultimate goal is the death of the emperor and the princess, 2 people who betrayed her, and in the emperor's case, murdered her father.