'Lady of Avalon' by Marion Zimmer Bradley (London, Penguin Books, 1998, ISBN 0-14-024193-0)
The story is divided into three periods: one from A.D. 96-118, another from A.D. 285-293 and the last from A.D. 440-452.
In the first part Avalon is still a place of this dimension and the Romans come marching in Brittania. Also, there is the rising amount of fanatic Christian monks, against heathens and pagan beliefs.
These two factors are a realistic threat to Avalon and the High Priestess Caillean puts Avalon apart from this world by calling magic mists around the islands of Avalon.
From 285 till 293 the new High Priestess Dierna must save Brittania from the Saxon hordes and in the last part Brittania seems lost forever, but the Druids and priestesses (Viviane, Igraine, Morgause etc.) of Avalon wait for the Defender, Arthur, and Merlin.
People who have read 'The Mists of Avalon' know that this is where that story starts off.
Apparently there is a third part in these series called 'The Forests of Avalon' but I haven't read that