The Wishsong of Shannara.

Horror stalked the Four Lands as the Ildatch, ancient source of evil, sent its ghastly Mord Wraiths to destroy Mankind. Only Druid Allanon held the magic power of wishsong that could make plants bloom instantly or turn trees from green to autumn gold. But she, too, was in mortal danger, and Ildatch waited for Brin to fall into his trap....From the Paperback edition.