Valley of the Lost (Deltora Quest 1 #7)

Lief, Barda and Jasmine, searching for the seven lost gems of the magic Belt of Deltora, have almost reached their goal. Six gems now gleam in the Belt, but the last must be found before Deltora can be freed from the tyranny of the evil Shadow Lord.
The companions have faced many terrors with strength and courage. Now they are about to meet dark mysteries that strength and courage alone cannot defeat. If they fail, their quest will be lost, and they will remain forever trapped in the swirling mists of the Valley of the Lost.