The Unicorn Hunters by Katherine Arden

Anne of Brittany was a child when France invaded and drove her royal father to his death. Now she is a young woman, sovereign duchess of an occupied realm, and France means to crown their conquest by marrying her to their king. Such an alliance would put her title, her lands, and her body forever in the hands of her enemies.

But Anne refuses to be the last duchess of Brittany.
Her only hope of resisting conquest is another alliance sealed with marriage, so Anne arranges a daring last gambit: a secret betrothal to Charles of France’s greatest rival. But secrets are hard to keep in a world where rival courts spy on each other with diviners.
The forest of Brocéliande was once the haunt of Merlin the Enchanter and the long-lost faerie queen. But magic is long gone from Broceliande, except for the occasional sight of a unicorn and one critical quirk: This ancient forest is completely hostile to divination.
While pretending compliance with France, Anne plans a unicorn hunt in Brocéliande. A bit of pointless pageantry. A diversion so she can wed in secret.
Or so she thinks.

In this rich and epic novel, the author of the acclaimed Winternight trilogy turns the real history of a remarkable woman into an unforgettable tale of mystery, enchantment, and the price of power.