Enemy of God.
I first discovered this tale on Audio book read by Tim Pigott-Smith. On reading this book I was amazed, the brilliance of the author shines through here as he described a tale of Arthur through the eyes of a spear man.
Derfel (the spearman), starts his tale looking back into the past as he writes it down on parchment for his Queen. He describes the years gone by and his involvement in saving Briton from darkness. He describes his life and the lives of all the significant characters he encounters, Arthur, Merlin, Galahad et al, and the role they played in Britons Liberation. The book also tells of Pagan druids and the birth of Christianity in Briton.
This book tells the tale of the search of the cauldron and Merlin's journey to collect the treasures of Briton. It also follows Arthur and roams its way to hard decisions and sadness.
This is the second book in a trilogy and I only wish there were more, the three books are excellent and the story cannot be left alone once this first book is finished. My only advise is order all three so avert disappointment through waiting.
Fiction yes, believable well, if Arthur did exist then this story of Arthur is the one I imagine.