Coalescent

Coalescent is a science-fiction novel by Stephen Baxter. It is part one of the Destiny's Children series. The story is set in two main time periods: modern Britain, when George Poole finds that he has a previously unknown sister and follows a trail to a mysterious and ancient organisation in Rome (Puissant Order of Holy Mary Queen of Virgins); and the time of Regina, a girl growing up during the ending of Roman rule in Britain, around AD 400.