Les Dames du Lac, Tome 4 : La Prêtresse d'Avalon

Blurb

Priestess of Avalon is a 2001 novel by Marion Zimmer Bradley and completed posthumously by Diana L. Paxson. It follows detailing the life of Helena, first wife of Western Roman Emperor Constantius Chlorus and mother of Constantine.
The Avalon series was started and became prominent from the titles published in Bradley's name, however the series is now being continued by Diana L. Paxson.