I envision this as sort of a regency, victorian, ballgown type of setting, but I think it could work in a few different eras
Accidental Empress/Queen
An illegitimate daughter, Cassandra never expected to marry well. She had been raised as a servant in her father's palace, hated by his wife and her half siblings, and expected to continue her life that way until she managed to escape, probably by becoming a servant in another noble house. She never expected to be thrust into the arms of a foreign emperor as a sacrificial lamb in place of her sister.
But when their small country is invaded and a treaty signed, their country becomes a vassal to the new Empire, her father is demoted from King to Duke, and her sister Selene is promised to marry the Emperor to solidify peace. But her sister does not want to marry an old, ugly, impotent man, rumored to be cruel and violent. She ran away with a young knight, leaving the family with the consequences. Leaving Cassandra to clean up her mess, just like she cleaned up Selene's room and all her other messes since they were children.
With an executioner's axe hanging over their heads if they didn't provide a daughter to be the bride, the family made the easy choice to sacrifice Cassandra in her sister's place, forging paperwork to say they adopted her as a child and sending letters faking sadness over sending her.
She met him at the wedding, and he was not what she had been told. Older than her but within an acceptable number of years, tall and obviously strong, a warrior, with dark hair, and a face that made her catch her breath. Now married, she doesn't know how to be an Empress, since she never had the formal training her legitimate sister did, and she doesn't know how to be a wife. But there is only one of those things that she can admit not knowing, and ask her new husband for guidance.
Soon, she finds her wifely duties a relief; excelling at something feels good, especially after long days filled with social and political failures, uncertain how to use her new status and power. She feels like a fraud and a failure everywhere but their rooms, and starts seeking out his attention.
Her new husband is… confused. Cassandra is not the spoiled princess he thought he would be getting, the rock around his neck that he took for the sake of peace. She is naive when it comes to court politics, her nature friendly and outgoing, though she is nervous. He can tell she is hiding something, but her reactions are honest. For man used to seeing pretense and betrayal in everyone who meets him, he has only seen the clouded look of deception in her eyes a few times. Only about her family.
She never lies in their bedroom. Her pleasure with his touch is genuine. And when she starts coming to him, begging for him to fuck her, or to let her please him in other ways, he cannot resist. She doesn't ask for political favors or allude to machinations. She doesn’t pretend to enjoy his presence. When they are alone in her rooms, and both their facades are gone, she likes him. Not for his power and position. She wants him the way a woman wants a man, with no ulterior motive, other than to make each other feel good.
He knows she feels uncertain of her place, though not way. He is happy to fuck her and guide her until she is confident as his Empress.
And when her family comes months later, asking him to divorce Cassandra and marry their real daughter? The truth of who she is comes out. But what will he do about it?
Your support makes Blue Moon possible (Patreon)