Bloodfire Quest: Book 2 of The Dark Legacy of Shannara by Terry Brooks
The quest for the long-lost Elfstones has drawn the leader of the Druid order and her followers into the hellish dimension known as the Forbidding, where the most dangerous creatures banished from the Four Lands are imprisoned. Now the hunt for the powerful talismans that can save their world has become a series of great challenges: a desperate search for kidnapped comrades, a relentless battle against unspeakable predators, and a grim race to escape the Forbidding alive. But though freedom is closer than they know, it may come at a terrifying price. Back in the village of Arborlon, the mystical, sentient tree that maintains the barrier between the Four Lands and the Forbidding is dying. And with each passing day, as the breach between the two worlds grows larger, the threat of the evil eager to spill forth and wreak havoc grows more dire. The only hope lies with a young Druid, faced with a staggering choice: cling to the life she cherishes or combat an army of darkness by making the ultimate sacrifice.