Two Twisted Crowns by Rachel Gillig

Book #2 in The Shepherd King series.

Gripped by a tyrant king and in the thrall of dark magic, the kingdom is in peril. 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’re going to find the card before Solstice and set free the kingdom, they will need to journey through the dangerous mist-cloaked forest. The only one who can lead them through is the monster that shares Elspeth’s head: the Nightmare.

And he’s not eager to share any longer.