Being a Green Mother by Piers Anthony, 313 pages

Orb had a rare gift-the magic which manifested whenever she sang or played her harp. No one could resist her music. But greater magic than Orb's lay in the Llano, the mystic music that controlled all things. The quest for the Llano occupied Orb's life.
Then she met Natasha. He was handsome, charming-and an even finer musician than she. Natasha began teaching Orb the music of the Llano, and she found herself falling in love with him.
Then Orb's mother Niobe came as an Aspect of Fate, bearing the news that Orb was chosen for the role of the Incarnation of Nature-the Green Mother. And Niobe warned Orb of the prophecy made years before-that Orb was to marry Evil. Could she be sure that Natasha was not really Satan, the Master of Illusion, laying a trap for her?