The "Winternight Trilogy" is a fantasy series written by Katherine Arden, set in medieval Russia. It follows the story of a young girl named Vasya Petrovna, who possesses the ability to see and communicate with spirits and mythical creatures from Slavic folklore. The trilogy explores themes of tradition, magic, and the struggle between old beliefs and the encroaching influence of Christianity. The series consists of three books: "The Bear and the Nightingale," "The Girl in the Tower," and "The Winter of the Witch." Each installment delves deeper into Vasya's journey as she confronts both external and internal challenges, including her family's expectations and the dangers posed by dark forces. The trilogy has been praised for its rich world-building and lyrical prose.