Elspeth is infected with magic, in a kingdom where that will get you killed. Sent off to live with her aunt and uncle, she’s kept away from court, but is drawn back in for the Equinox celebrations.
The only official magic allowed are Providence Cards, created by the Shepherd King, who sought to protect the kingdom from the Spirit of the Wood, but in the process lost his soul. Elspeth’s great secret is that her magical infection allows her to draw power from the cards, and she’s drawn power from the Nightmare.
No one at the court is who they appear, and Elspeth herself finds that everything she thought she understood about her power was wrong. This book ends on a bit of a cliffhanger, and I’m very interested to see where things go from here.