To begin, I'd like to say that the Pendragon Cycle is a brilliant work, and the other books in it stand without this, frankly, unnecessary work.
I am not sure what prompted Stephen Lawhead to write a fifth volume, but all I can say is that it simply feels wrong - almost as if someone else wrote the book.
The traditional tales of King Arthur ends with Arthur disappearing mysteriously, an event depicted magnificently at the end of Pendragon Cycle 3: Arthur, as Arthur sails off to Merlin's Grandfather's palace on Avallon - a secluded island (marked as the Isle of Wight on the map at the beginning).
So it came as a bit of a surprise to me to discover that this book begins with Arthur, Merlin and others not only returning from Merlin's Grandfather's palace, but somehow the palace Arthur was returning from was at Glastonbury Tor, the location Avallach leaves in the previous book.
The structure of the book was also different to the other books - while the others are all split into three or four sections, this is one single section told through the eyes of one person.
There were other discrepancies too, and the narrative did not seem up to the standard I have come to expect from Lawhead. To be honest, I'd recommend avoiding this book, because it kind of spoilt the rest of the series for me.