Marion Zimmer Bradley's Sword of Avalon by Diana L. Paxson is the tale of how the Sword from the Stars(Excalibur) was first created long before it ever came into the hands of King Arthur, and the life and ordeals of the man destined to wield the sword as he unites the tribes of ancient Britain against an evil warlord. This story takes place about a thousand years after Ancestors of Avalon and two of the characters are specified as being reincarnations of characters from the previous book, though it is indicated that they had reincarnated multiple times in the intervening years. It's not necessary to read The Fall of Atlantis and Ancestors of Avalon before reading this book, but it will help in understanding the history of these characters.
While I did enjoy reading this book, it was unfortunately not as good as Ancestors of Avalon and I only rated it three stars. It was slow paced like the previous two books that I had read, but in this case the slow paced bits were rather tedious and at times I wasn't sure what they added to the story. After reading The Fall of Atlantis and Ancestors of Avalon I had this strong craving to read more and I just didn't get that feeling with the Sword of Avalon. It's really a shame because the story idea is an excellent one and could have been done so much better. I would still recommend this book to fans of the series, but just don't expect it to be quite as good as the previous books mentioned.