Once Upon a December: The Girl in the Tower by Katherine Arden

Katherine Arden’s The Girl in the Tower picks up right where The Bear and the Nightingale left off, with Vasya Petrovna on the run with her magic stallion Solovey. Disguised as boy—respectable girls cannot travel safely alone in 14th century Russia—she is determined to see the sea and explore the world. Along the way she collides with a gang of well-equipped bandits burning villages to the ground and kidnapping young girls.

At the same time, Vasya’s brother Aleksander, now a highly respected monk in Moscow, and Grand Prince Dmitrii set off to investigate the raids at the behest of a strange new boyar calling himself Kasyan. The men’s path soon intertwines with Vasya’s, making her gender-bending ruse all the more risky.

Further complicating matters is her growing bond with Morozko the frost demon. Neither of them quite know where their relationship is headed, but whatever the destination, the journey there ...