Two Twisted Crowns by Rachel Gillig

Elspeth and Ravyn have gathered most of the twelve Providence Cards, but the last, and most important one remains to be found: The Twin Alders. If they are going to find it before the Solstice and cure the kingdom of the dark magic infecting it, they will need to journey beyond the dangerous mist-cloaked forest that surrounds their kingdom. And the only one who can lead them there is the monster that shares Elspeth's head. The Nightmare. And he's not eager to share any longer.