This is an elaborate set up, but bear with me.
She was the princess of a faraway land, but despite her position of authority, she grew up in a miserable situation. She was raised by her father, who saw her as a means to political gain, nothing more. She was little more than a servant in her own home. On her 18th birthday, she was promised to Prince James Nolan, the leader of a mighty nation to the far north. She expects her husband to be to be as petty and self absorbed as her own family, but is surprised to find him considerate and mindful of the suffering of those around him. Despite an awkward start, the two fall in love. When they actually marry, it is of the princess's free will.
In the first few years of their marriage, the two of them make great strides towards improving the lives of their people, as well as helping their surrounding nations. They even save the lives of the Anar, a nomadic group of travelers with powerful magical abilities. In gratitude, the Anar promise Henri one favor, though his fear of dark magic stops Henri from ever cashing in.
On the evening of their third anniversary, she returns home to find her husband dead, stabbed in his sleep. His father, the king, has also been murdered. The prince's cruel brother assumes the throne and starts to lead the entire continent into a new dark age. He means to rule the known world, and his efforts have created widespread disease and famine.
Desperate and forlorn, the princess goes to the Anar and begs to cash in the favor that was owed to her husband. Her hope is that the Anar can use their magic to bring James back. Unfortunately, the Anar cannot defeat death, but they can cheat it. The Anar offer to send her back in time to the day she first met her husband. From there, she can prevent the tragedy that tore her home apart. Unfortunately, there are several complications.
1. The same person cannot exist in a timeline twice. To attempt this, her identity as a princess would be destroyed. She would look the same, but her royal lineage would be gone. She would have a completely new history, and a version of her old self wouldn't exist in this timeline. The Anar can't control who she comes back as, only that it will be someone close to James.
2. The magic manipulating fate like this is fragile. If she says who she is or tells anyone of the future, it will be destroyed and everything will go back to the way it was. (Perhaps getting James to say that he loves her again will break this part, but that can be decided)
She has little choice but to accept the deal, and history changes around her. She quickly discovers that this new timeline is darker than the last one, as she is a slave being sent to the castle to work. Slavery was never a factor in her timeline. Most of the incoming slaves are sent to the lower levels to work, but she is stripped, tied, and gagged before being left in an all too familiar bed. She has been selected her to be the personal pleasure slave to Prince James, a way to ease his mind in these troubled times. The door opens and her former husband enters, though he has no memory of their history together.
I'm picturing this as a careful balancing act. James doesn't want a slave, but if he sends her away, he will die like he did before. The complexity is what makes it so appealing.