The Winter King by Bernard Cornwell

First book in the Warlord Chronicles trilogy, in which we discover Arthur as a fully-fledged warrior through the eyes of one of Merlin's pupils, former Saxon-slave Derfel, who will become one of Arthur's trusted lieutenants. From the author's website:

"‘Once upon a time, in a land that was called Britain, these things happened . . . . and I was there, and this is how it was.’ The Winter King, like the rest of the trilogy, is narrated by Derfel (which is pronounced Dervel), one of Arthur’s warriors. This first book tells how after the death of Uther, High King of Britain, the country falls into chaos. Uther’s heir is a child, Mordred, and Arthur, his uncle, is named one of the boy’s guardians. Arthur has to fight other British kingdoms and the dreadful “Sais” – the Saxons – who are invading Britain. Arthur is supposed to marry Ceinwyn, a princess of Powys, but falls disastrously in love with Guinevere – ‘There have been many more beautiful women, and thousands who were better, but since the world was weaned I doubt there have been many so unforgettable as Guinevere . . . and it would have been better, Merlin always said, had she been drowned at birth.’"

In this series Arthur is already a grown, full-fledged warrior with quite a reputation! He is Uther and Igraine's son, but a bastard! Mordred, Uther's legitimate son and heir, died shortly before this book begins, leaving behind a pregnant wife. The story starts with the birth of Mordred's posthumus son, also called Mordred, born deformed (with a club foot) but which Uther says won't stop him from ruling or fighting from horseback. Uther's final months are trying to ensure the stability of his realm and his grandson as his heir (to the kingdom of Dumnonia, not as High King of Britain since he must win that right in battle). In the meantime Mordred (and his mother) are being cared for in Merlin's household of Ynys Wydryn by Morgan, Arthur's widowed sister, one of Merlin's priestesses. 

Shortly before his death, Uther calls a High Council during which he has his fellow kings and lords swear to uphold his grandson Mordred as King of Dumnonia after him. As part of this the one-time enemy King of Siluria, Gundleus, is betrothed to widowed Norwenna, Mordred's mother. He also names four sworn protectors, men bound by death-oaths to defend Mordred's life with their own: King Tewdric of Gwent (Uther's strongest ally), Owain Champion of Dumnonia, Merlin Lord of Avalon and Arthur (that one was pretty much forced on Uther who hates his bastard son). Shortly after Uther's death betrayal of course ensues by the hands of Gundleus, which Morgan and Nimüe foresaw, so the traitor was presented with a different baby (which he killed right after killing Norwenna) while they escaped with Mordred and made their way to Uther's stronghold for protection. And thus we have the usual tale of someone mistakenly killing another child in Mordred's place!

This is the tale of Arthur's rise to power, albeit as the protector of his nephew's realm and not as King himself. It chronicles the confrontation between political rivals inside the kingdom of Dumnonia, as well as the battles against other British kingdoms both to survive and with the hope of establishing a lasting peace between the kingdoms so they can face the invading Saxons as a unified Britain. We also get some action "overseas" with fighting against the Franks invading Benoic (Brittany) which as Warlord Arthur was sworn to protect (Ban only "loaned" him to Dumnonia for a short while, he was supposed to go back!) and meet King Ban, his wife Elaine, the heir Lancelot and his brother Galahad.

We also see what seems to me shall be the major conflict theme here (other than the obvious fight against the Saxons): religion! Followers of the "old religion" (i.e. that of the Druids before the Romans came) believe it is their last chance to reclaim Britain for their gods, or lose it forever to the Christian god. So we've got Christians vs pagans and then Britons vs Saxons and of course people fighting for power. But it appears that Christianity will be part of Arthur's undoing.

Elements from Arthurian Legend:

• Arthur
• is a great Warlord and not King (bastard son of Uther), and dedicated to both defending his nephew Mordred's kingdom and uniting the British realms to fight against the Saxon invaders.
• is not a defender of the Christian faith (as in the medieval legends and many versions of the story), in fact it seems Christianity (or at least many of its representatives) is one of Arthur's main foes!
• falls madly in love with Guinevere upon seeing her and by marrying her breaks his betrothal to another woman (provoking a war)
• wields the "magical" sword Excalibur given to him by Merlin

• Merlin
• is a Druid with supposedly very strong magical powers, and yet we never actually witness him doing any magic.
• he is Arthur's supporter, but not his close councillor. In fact we rarely see him!
• is more interested in reclaiming the British gods (which have been fading into the background since the Roman invasion) than in getting rid of the Saxons. For this he is on a quest to find the "Knowledge of Britain".

• Death of a child in an attempt to kill Mordred.
• Morgan: Arthur's sister (full-blood this time) and a powerful woman as one of Merlin's priestesses. Faithful to her brother.
• Nimüe: Merlin's lover and also a powerful priestess (replacing Morgan as Merlin's #2). As in other tales much younger than Merlin (here linked by love and childhood friendship to our narrator Derfel).
• Mordred King! But still only a baby while Arthur is in power.
• Guinevere: as usual a great beauty and Arthur's wife, for whom he basically sacrificed a chance of peace and a unified Britain in order to marry! (he was engaged to the daughter of the rival king of Powys)
• Lancelot: son and heir of Ban of Benoic, but not the great hero of legend! He's made out to be a coward and an opportunist who has the bards compose great songs about his heroic feats. Incredibly handsome and charming.
• Galahad: Lancelot's 1/2 brother instead of his son, but still the better warrior and better man.
• Tristan of Kernow (Cornwall): son of King Mark instead of his nephew. His father just married his 5th (?) wife... no other mention of Isolde yet. ;o)
• Other legendary warriors mentioned: Bors, Cei, Gereint, Melwas, Owain, Sagramor (a black Numidian!)
• Balise: an old druid, once Merlin's teacher (reference to Blaise?)
• Saxon leaders: Aelle, Cerdic