ONE DARK WINDOW: The Shepherd King (Book 1) by Rachel Gillig

One Dark Window by Rachel Gillig is a dark, gothic fantasy set in the mist-locked, eerie kingdom of Blunder, where magic is an infection and those who possess it are hunted. Protagonist Elspeth Spindle carries a dangerous secret: an ancient, mercurial spirit known as the Nightmare trapped inside her mind. While the Nightmare protects her and keeps her hidden, nothing comes without a cost, and his grip on her sanity grows tighter by the day.

Elspeth's life is upended when she crosses paths with a mysterious highwayman who turns out to be the King's fugitive nephew and Captain of the Destriers, currently guilty of high treason. Driven by a desperate need to save the kingdom, the two form an alliance to track down twelve magical Providence Cards before the winter Solstice. As they race against time and face mounting perils, their growing attraction is threatened by the terrifying reality that the Nightmare is slowly taking over Elspeth's mind, creating a high-stakes race where failing to find a cure means losing herself completely.