Who is Joan of Arc from The Magician and what is their importance?

Joan of Arc was the famous teenager, who saw visions of God that told her to take back her home country from the English. She was eventually burned at the stake. However, in this novel, Scathach claims that she saved Joan from the stake and then gave her blood to help heal her injuries that resulted in Joan becoming immortal. Joan and Germain have been married for four years when this novel begins.