Under A Velvet Cloak

Nox, the Incarnation of Night and Mistress of Secrets... It is the year 500 AD. Kerena, a beautiful young girl living near King Arthur's Camelot, is apprenticed to Morely the Seer. Morely teaches her fantastic magic, but when he mysteriously disappears, Kerena finds herself out on the street and must resort to prostitution to survive. Kerena prospers as a skilled courtesan until she is taken into service by Morgan le Fey, the sorceress sister of King Arthur. Kerena's knowledge of magic grows as she is required to carry out nefarious deeds for the evil Lady Fey. One of her missions is to seduce Sir Gawain, Knight of the Round Table, to prevent him from locating the Holy Grail. But Gawain and Kerena fall in love and she conceives his child, a child she discovers is cursed to die an early death. Using her Seer abilities, Kerena tries to locate the fabled Incarnations of Immortality, seeking their aid in removing the taint from her baby. Rebuffed by all seven major Incarnations, Kerena vows revenge. But revenge against such mighty immortals would require an even greater power... (Book 8 of Piers Anthony's Incarnations of Immortality series.)