Excalibur by Bernard Cornwell

We've reached the final volume in the Warlord Chronicles trilogy, with Arthur finally the undisputed leader of the British (while his nephew Mordred remains King of Dumnonia in name only, guarded in his palace with no real power, kept alive basically until he marries and produces an heir). It's time for one of the key events in Arthurian lore: Mount Badon! From the author's website:

"If Arthur existed (and I am quite certain he did) then he was probably the great British war leader who won the battle of Mount Badon. No one knows where it was fought, or how it was fought, but we do know that the battle took place and it was the one great defeat inflicted on the English invaders of Britain. In Excalibur we follow Arthur and Derfel to that enormous struggle and incredible victory. It not only throws the Saxons back, but reunites Arthur and Guinevere. He might hope now to be left alone, to have a time of peace after gaining a great victory, but new enemies arise to destroy all he has achieved. First is Mordred himself, the crippled king who owes everything to Arthur and now tries to kill his benefactor. Mordred’s ally is Nimue who has come to hate her mentor, Merlin. And so the story ends as it has always ended, at Camlann . . . ‘and so my lord was gone. And no one has seen him since.’"

And so the end of Arthur's story begins. It begins with Samhain Eve (currently known as Halloween), a night for powerful magic. A night when Merlin and Nimüe are finally going to summon the Gods of Britain so they may chase away the Christians and the Saxons. It's the culmination of Merlin's life, of his ongoing quest for the "Knowledge of Britain" and the thirteen Treasures of Britain: the Basket of Garanhir, the Horn of Bran Galed, the Chariot of Modron, the Halter of Eiddyn, the Knife of Laufrodedd, the Whetstone of Tudwal, the Coat of Padarn, the Cloak of Rhegadd, the Dish of Rhygenydd, the Throwboard of Gwenddolau, the Ring of Eluned, the Sword of Rhydderch (a.k.a. Excalibur) and the Cauldron of Clyddno Eiddyn (which they obtained at the beginning of Enemy of God). Only these last two had any intrinsic value, the others all falling to pieces... as if they were a sign of how far away the Gods had gone. The ceremony requires fire, of course, and that most shocking of all Druidic rituals: a human sacrifice, of royal blood. But things aren't allowed to take the complete course that Nimüe would have wanted (she was already worried that Merlin's "emotions" would get in the way) and this will effectively turn Nimüe into Merlin and Arthur's ennemy! Merlin because from this day on she despises him for weakness (this will lead to Merlin's "end" as we know it from legend, held captive by Nimüe so she can obtain all his secret knowledge), and Arthur because among other things she will want to get Excalibur back from him to try it all again. Poor Arthur. In the last book he was already considered the "enemy" by most Christians, and now most of the more religious British Pagans (following Nimüe's lead) consider him their enemy as well! Religion is definitely not his ally in these tales...

Moving into the section relating to the battle of Mount Badon. Arthur sets all his plans in place, knowing the Saxons will invade after the spring equinox, and worried about not having enough troops to fight them since the spiteful king of Gwent won't join in saying the Saxons won't attack his kingdom because God (he's Christian) will protect them. Arthur's plan is to tempt the Saxons to attack one single location and crush them there, but everything goes awry because a traitor in their midst informs the Saxons of the trap changing everything! Which eventually leads us to Arthur's most famous battle at Mount Badon. A battle so well written that I couldn't tear myself away from the book even though it was late in the night! Unfortunately for Arthur, to get the support he needed (namely from Gwent) for this enterprise he agreed to a few conditions... the principle being that he let Mordred effectively reign in Dumnonia while he (Arthur) retired to Isca (Caerleon!) in Siluria (under the watchful eye of Gwent).

So as a reward for Arthur's greatest triumph he gets exiled (in a way, he kind of loves his new life!) and Mordred finally takes control of his kingdom. And it's all downhill from there for Dumnonia. His hatred and enmity of Arthur will lead to his persecution of him, aided by Nimüe. Arthur and his closest people will have to flee to the southern coast to find a ship Merlin had ordered stand ready to receive him (Merlin's "Last Enchantment" and gift to Arthur, prepared before Nimüe got to him), hoping to flee to Armorica (Brittany). It's time for the famous final battle between Mordred and Arthur on the beach at Camlann, when one dies and the other is mortally wounded to be taken away by ship to a destination unknown: "And I watched and wept, my arm around Ceinwyn, as the pale boat was swallowed by the shimmering silver mist. And so my Lord was gone. And no one has seen him since."

Elements from Arthurian Legend:

• Arthur
• Final victory over the Saxons at Mount Badon... then effectively "retires"! Well, is kind of forced into it at least (but he likes it)
• Takes a second wife!!!
• Ends pretty much the same as in most tales, and sails away to lands unknown.
• Merlin
• Is over 80 years old.
•  Has two key moments: during the ceremony on Samhain Eve, and at Mount Badon, then pretty much fades away.
• Is kept prisoner by Nimüe until she can learn all his secrets and magic and use them against Arthur.
• Bors -> Lancelot's cousin and champion who was at his side during the rebellion, but now rejoins Arthur so as to not have to join Saxons in killing Bretons at Mount Badon. 
• Gawaine -> Finally shows up! Arthur's nephew by sister Anna and the king of Armorica (and not Morgan or Morgause). Was in part tutored by Merlin so he might play a big part in the summoning of the Gods.
• Guinevere -> Is locked away in a nunnery, guarded by Morgan (they can't stand each other), but ends up participating at Mount Badon in an astounding way and is redeemed! 
• Lancelot -> Still a traitor, joins in with Cedric and the Saxons to attack Arthur.
• Mordred -> Finally gets a taste of battle at Mount Badon, and discovers he likes it! Keeps fighting wars once he gets to "really" be king of Dumnonia ('cause Arthur is forced into retirement), mostly fighting the Franks for his uncle in Armorica. This gives him the training and the troups to confront Arthur in the end. 
• Nimüe -> Has completely gone over to the dark side! Now takes Morgan's place from the Legends as Arthur's enemy from the old religion, and is still the one responsible for Merlin's death after imprisoning him in a cave and "stealing" his knowledge.
• Taliesin -> Not usually a part of Arthurian Legends (except as a historical figure in Britain's distant past), the greatest bard that ever was. Here has an integral part to play in Arthur's final tale.
• Excalibur -> Is returned (briefly) to Merlin for the summoning of the Gods. After his final battle at Camlann Arthur asks Derfel to throw it in the sea so Nimüe will never get it.