This is actually several different ideas, all centered around the idea of someone royal suffering an extreme change in station. These are all rough ideas that would need to be fleshed out.
1. The Bride
Two kingdoms have been locked in a ten year war. At long last, one kingdom has crushed the armies of its rival. The victorious king is young, having replaced his father when his family was killed in the war. As part of the peace signing, the young king demands wealth, land, and several artifacts. The biggest blow to his rival kingdom, however, is the demand that the princess from the other kingdom marry him. She will be queen, but the position will have no power. Instead, she will be little more than a slave, expected to bend to the king's every whim. I imagine this story would have a good deal of bondage and submission, as someone who comes from power is forced to give that power up.
2. The Price
A young princess has been forced to watch as her kingdom is slowly killed by a deadly plague. It destroys the crops and decimates the population. If things keep going as they are, her people will die out.
Exactly one year ago, the princess wrote a letter to the magi, a small collection of mages who generally don't involve themselves in the affairs of kings and kingdoms. The odds of getting a mage to help them were remote, so she is surprised when a young man with the powers of a mage arrives, assesses the situation, and assures her that he can resolve the matter. His only price is that she must give herself to him. They will leave her kingdom and she will serve him in any capacity he desires along his travels.
(This one would play a lot like the first one, but it would have magical elements and would involve a less angry master)
3. The Darkness
Her marriage was arranged when she was a child, and ever since then she has heard nothing about how wonderful her husband to be is. They tell her the prince is handsome, kind, and fair. At their initial meeting, everything seems to be going smoothly, and he seems to be everything that was promised.
Shortly after the first meeting, the princess starts to have dreams that her prince has been touched by some great darkness. Something powerful resides within him. The prince seems to have the ability to cloud the minds of others, compelling them to do as he wishes. For some reason, the princess is immune to such things.
To the prince, as well as the darkness within him, she represents the perfect challenge. She is the first being he has encountered he cannot control with his will. To the outside world, he is determined to present a loving couple, but behind closed doors he is determined to bend her to his will.
(This one would be a little more complex. I imagine there would be some sort of spell that forbids the princess from talking about the prince's true nature. She would be unable to warn others as the prince worms his way into the minds of her court and starts to use his unquestioned power to make the princess a prisoner.)