Nightwings - Robert Silverberg

Only at night, on the winds of darkness, can she soar. And it was Avluela the Flier's ebony and scarlet wings that led the Watcher to the seven hills of the ancient city from which, in a moment of weakness, the Watcher failed his vigil, leaving the skies and deep space unguarded. The invaders came and conquered. With Avluela lost in the turmoil of conquest, the Watcher set out alone for the Holy City - home of the Rememberers, keepers of the past, and where the secret of Earth's salvation lay hidden in antiquity. On his journey, the Watcher hoped to recapture his youth and find the soaring, beautiful woman he loved. But Avluela held more for the Watcher and Earth than love. Her wonder stretched beyond flight, for she knew the riddle to free all men....