The kingdoms of Entimir and Erebor had always been close. 20 years ago, they arranged a marriage between the prince of Entimir and the princess of Erebor when the two came of age. It would have ushered in an even greater era of peace, except Erebor was growing resource starved and needed to expand. Five years ago, the king of Erebor made deals with several of its neighbors to attack Entimir and split its resources. The plan went wrong and resulted in several years of war.
Now the war is over, and Erebor is on the verge of absolute collapse. To formalize the peace, Entimir demands gold, land, and several famous relics. Worst of all, however, the king of Entimir demands that the marriage that was arranged 20 years ago be gone through with, as both children have come of age. The king brags to everyone who will listen about how how the girl will be a captive, paraded about, chained, and used to humiliate her father at every turn. It is well known that the king of Erebor loves his daughter more than anything.
No one could have realized just how far the king would go to protect his daughter though. Rather than turn the girl over, he searches his kingdom for anyone who looks like his daughter. He finds someone almost perfect, a slave girl. The girl is dressed like a princess and handed over to their rivals, warned that should she reveal that she isn't the true princess, her new family's fury would be immeasurable. She would be killed horribly.
The wedding takes place, and the 'princess is shackled, forced to wear clothing her father in law picked out, and displayed as a symbol of Entimir's power. When she reaches the bedroom with her new husband, she cracks and tells him the truth. Rather than face death, the prince just holds her and assures her everything will be alright, somehow.