Mirror of Destiny — Andre Norton

The King's lottery has determined that Twilla, young, orphaned apprentice of a renowned wisewoman, must marry -- for only the wedded can survive the terrible fate awaiting those who penetrate the primeval forest. Altered by a talisman of great power, she escapes her unwanted lot -- joining a commander's tragically blinded son on a remarkable journey from peril to peril. For they are the chosen who must rescue the vanquished of an ancient war of magicks. . . and shape the destiny of a bloody, disputed land.