One Dark Window

Elspeth Spindle needs a monster — an ancient spirit she calls the Nightmare. Only he can help her survive in the kingdom shrouded in magical mist. He protects her. He keeps her secrets. He is locked in her head.
Elspeth's life changes when she meets Ravin Yu — the king's nephew, captain of the elite soldiers… A traitor acting behind his powerful uncle's back.
Together they intend to collect the twelve Cards of Providence — a powerful magical deck that may be the key to freeing the kingdom from the creepy mist and deadly plague.
But the stakes are rising, and the Nightmare is slowly and inevitably taking over Elspeth's mind. And she has little time left to stop him... and save herself.