Hearts Lost by Meredith Hart

I was sent to stop the monsters. I’m trapped with one of them instead.

In exchange for my sister’s freedom, I have to enter the Lands Below, stop their attempts to bring down the barrier that keeps them all trapped underground, and kill Princess Elanerill. There’s no way this ends well, but I don’t have a choice. Not if I want my sister to make it out of King Grathgore’s court alive.
But my borrowed magic backfires, knocking me out of the Lands Below and into a forest filled with strange shadow animals where the ground itself shifts beneath my feet.
And I’m not alone. Aloserin, the dangerous Royal Guard from the Lands Below who’s trapped with me, calls this place the void. He claims those who enter the void’s twisting forests and shifting paths rarely return. Still, I have an advantage those previous travelers did not: I can communicate with the shadow animals that so terrify Aloserin. Perhaps the two of us can survive long enough to find our way out.
As long as Aloserin doesn’t discover that I’m the reason we’re both trapped here.