Complex sentence: When the princess tries to prevent him from leaving, as she thinks that he is the assassin, he knocks her out.Fortunately, the assassination attempt failed, but Dae Joyoung is cut by one of the poisoned blades and falls unconscious.
Split into simpler sentences: When the princess tries to prevent him from leaving, as she thinks that he is the assassin, he knocks her out. The assassination attempt fails, thanks to Dae Joyeong, but he is cut by one of the poisoned blades and falls unconscious.