Power. The Endonshan Chronicles, #3

When Princess Tashan summons Mara and her husband, Mikell, they never thought they would be sent on a new adventure with their old friends Ari and Korent. The four venture deep into the mountains to hunt down a powerful stone with the ability to amplify a magic user's powers. They have to face not only the perils of the journey into the mountains - cave-ins, sheer cliffs, and the vicious underdwellers - but also tensions between them as Mikell grows increasingly more alarmed at Mara's changing personality.
When Mara is separated from the others, she has to rely on herself to survive and find her friends even as they strive to find her. Not only that, but they discover someone else is after the stone: an enemy who will stop at nothing to get it and use its potential to become a violent, destructive force.