Final Fantasy IV/Return to Baron

Fight Yang
Exit the building of the Serpent Road and you will be back in Baron. Go to the Inn in Baron and climb to the top floor. There you will find Yang. Speak to him, and he will engage you in battle. Simply attack a few times, survive the battle and Yang will surrender, and join your party having been under mind control. He will also give you a key to a passageway to lead you into Baron Castle.