Book The Unicorn Hunters — Katherine Arden

Now Brittany is occupied by her enemies, her treasury empty, and only one thing is lacking to complete her realm’s subjugation: she is required, on pain of the sword, to marry the King of France.
But Anne cannot. She has promised her dead father that Brittany would never be conquered.
Defiantly, she betroths herself in secret to France’s greatest enemy. But in a world where courts may spy on each other by magic, there is only one way to solemnize this illicit union.
Under the guise of a hunting party, Anne takes her court deep into a deep forest, a strange place dogged with rumours of ancient enchantments; a place where diviners cannot see. She tells the French that she had gone there to hunt unicorns.
It’s a ruse, a lie, a feint.