The Original Shannara Trilogy #2 The Elfstones of Shannara Terry Brooks(2000)

The magical Ellcrys tree is dying, loosening the spell that bars the Demons from enacting vengeance upon the land. Now Wil Ohmsford must guard the Elven girl Amberle on a perilous quest as she carries one of the Ellcrys' seeds to a mysterious place where it can be quickened into a powerful new force. But dark on their trail comes the Reaper, most fearsome of all Demons, aiming to crush their mission at any cost.