Ok, I'll admit up front that I prefer Jack Whyte's Camulod series. But honesty compels me to admit Cornwell's version is probably closer to the truth.
It's comprised of three novels
The Winter King
Enemy of God
The Winter King
Uther the High King of Britain is dying. He blames his bastard son Arthur for the death of his beloved legitimate son Mordred, and is eagerly awaiting the birth of his grandson and heir Mordred. Mordred is born with a club foot, but is declared Uther's heir all the same. His mother is married off to King Gundleus, who kills her and attempts to kill the baby Mordred. Morgan (Arthur's sister) has substituted a slaves baby for Mordred, allowing Morgan, Nimue, Derfel and baby Mordred to escape from Gundleus' clutches.
Derfel joins Arthur's men and is put under the command of the King's Champion Owain. When Prince Tristan brings Owain's treachery to light Arthur fights Owain and wins. Derfel is then sent to Benoic. Galahad (King Ban's bastard son) and Derfel fight mightily to protect Benoic, they rescue Merlin, and barely escape with their lives when Benoic falls. Ban's wife (Elaine) and his heir Lancelot (boo hiss) escape before there is any real danger. But Prince Lancelot who gets to Arthur's court first, lies about his part in the fight.
Arthur was pledge to marry King Gorfyddyd's daughter Ceinwyn in order to bring peace to Britain. He breaks his betrothal to marry Guinevere, and starts a war instead. Deserted by the majority of his allies Arthur and his men face defeat at Lugg Vale. Prince Tristan brings his men against his father King Mark's wishes, but the tide is not turned until Merlin gets the Irish King Oengus Mac Airem to fight for Arthur instead of King Gorfyddyd. The book ends with Arthur's unexpected victory at Lugg Vale.
Enemy of God
Arthur plans to marry Ceinwyn to Prince Lancelot and give him Siluria to rule. Merlin promises Derfel his heart's desire if he will go with him to find the Cauldron of Clyddno Eiddyn (the forerunner of the Christian Holy Grail). Nimue has been talking with Ceinwyn and encouraging her to follow her heart. As a result at the betrothal feast Ceinwyn chooses Lord Derfel instead of Prince Lancelot.
Nimue, Ceinwyn, Derfel and Merlin face much danger but eventually find the Cauldron. Then Derfel joins Arthur to fight the Saxons. Humiliated by his inability to join the Cult of Mithras Prince Lancelot becomes a Christian. Arthur (who Guinevere encourages everyone to call a Prince) and Lord Derfel go to fight the Saxon King Aelle, leaving Prince Lancelot behind to defend their border. After Arthur defeats King Aelle, Prince Lancelot and the Saxon King Cerdic show up, forcing Arthur to make a peace with King Cerdic he doesn't want, and giving Lancelot the kingdom of Belgae, increasing his power in Britain. Lord Derfel is appointed the King's Champion, by Arthur.
Prince Tristan falls in love with his Step-Mother Queen Iseult. They flee to Dumnonia for protection, but Arthur turns them over to King Mark. Knowing that King Mark is a Tongued One, Prince Tristan chooses the Court of Swords. Culhwch (Arthur's cousin) and Lord Derfel offer to fight King Mark's champion in Prince Tristan's place. Arthur allows Culhwch and Lord Derfel to be trapped behind a shield wall, while Prince Tristan is slaughtered. Lord Derfel ends his friendship with Arthur over his betrayal of Prince Tristan.
Mordred comes of age and is named King. Culhwch challenges his right to rule. Lord Derfel refuses to kill his old friend when King Mordred demands it. Culhwch flees to King Cuneglas' kingdom. Arthur and Lord Derfel patch up their friendship. King Mordred sends Arthur and Lord Derfel to arrest Ligessac in hopes that the Christians will kill them. Arthur starts home with Ligessac as his prisoner, and Derfel goes to find his Mother, Erce. Erce tells him that King Aelle is his Father. Lord Derfel gets back to Dumnonia in time to see Lancelot wed the DEAD Norwenna in order to claim the kingdom of Dumnonia, and learns that King Mordred is dead. Arthur and Lord Derfel are also believed to be dead. Lord Derfel hurries home where his family is under attack by Prince Lancelot's Druids. His youngest daughter Dian is killed, but the rest of the family escapes to Cuneglas' kingdom. Galahad in the meantime has found King Mordred, and brought him to safety. Lord Derfel and Arthur join up to rescue Guinevere. Arthur discovers that not only is Lancelot her lover but she has used the Cult of Isis to put him on the throne. Lancelot is defeated and Arthur imprisons Guinevere. Arthur insist that Mordred is still King of Dumnonia, so they name Arthur Emperor, Lord of Kings.
Merlin has assembled the 13 Treasures of Britain and intends to summon the Gods on Mai Dun at Samhain. He is accompanied by Nimue and Gawain (who Merlin taught as a child to remain pure). Arthur, Galahad, Lord Derfel, Culhwch, Cuneglas and Issa storm Mai Dun to rescue Arthur's son when they realize Merlin plans to sacrifice him. They get there in time to save Gwydre (Arthur & Guinevere's son), and Mardoc (Mordred's son), but Gawain has already been sacrificed. When the God's don't come the people blame Arthur.
Arthur sends Lord Derfel to speak with King Aelle. Lord Derfel is forced to fight King Cerdic's champion, Liofa, before he can speak with his father. He leaves Liofa alive but scared. King Aelle gives Lord Derfel a ring to give to Ceinwyn and promises the wearer of the ring and her family will live when the Saxon's come.
Warned of Arthur's plans, by Mordred, the Saxons come at Beltane, surprising the Britain's. Lord Derfel, his family and Guinevere set out to join Arthur at Corinium. They are trapped by the Saxons at Mynydd Baddon. The Saxons believe Arthur is with them so they lay siege, and send for reinforcements. Looking at certain defeat Lord Derfel allows Guinevere to carry out her plan. She sends two fiery wagons toward the Saxon army saving the day. The next day the Saxons come in columns to prevent more wagons from wrecking havoc on their army, but Arthur shows up in time to prevent Mynydd Baddon falling to the Saxons. Liofa kills Cuneglas in single combat. When it looks as if the Saxons will win, Merlin shows up with Gawain (yes, I know he is dead - you'll have to read the book to find out) giving the victory to the Britain's. Severely wounded King Aelle begs Lord Derfel to give him a warriors death. Merlin warns Lord Derfel that Nimue will try to bring back the Gods and that she is harder and more ruthless then he is. Before leaving he tells Lord Derfel that what he seeks is among the dead trees. After searching the dead trees Lancelot is found and hung like a common criminal. Guinevere and Arthur reunite, and move to Siluria. Arthur's discarded wife Argante is married to Mordred. The troops of Dumnonia are split between Sagramor and Lord Derfel ( So everyone lives happily ever after - unfortunately we haven't reached the end of the book).
Gwydre (Arthur's son) and Morwenna (Lord Derfel's daughter) marry and have children. Mordered goes to Armorica to fight the Franks, where he is trapped and supposedly dying. Guinevere plots to give the crown to Gwydre while Meurig plots to take the crown for himself. Arthur sends Lord Derfel to Dumnonia to raise Gwydre's standard. Lord Derfel finds his garrison slaughtered by Mordred's forces and he is taken prisoner. Taliesin (a famous bard) poising as a Druid frees Lord Derfel and Bishop Sansum (Morgan's husband). Lord Derfel arrives home only to find Ceinwyn seriously ill. Nimue summons him and reveals that she has placed a curse on Ceinwyn and that she will not lift it until Lord Derfel brings her Gwydre and Excalibur. Nimue has blinded Merlin and is holding him prisoner. Merlin mangages to reveal to Lord Derfel the four curses Nimue placed on Ceinwyn and tells him that out of love for Arthur and Derfel he has a final gift for them. When all is lost they should go to Camlann and find Caddwg the boatman and he will take them into the silver mist. Lord Derfel returns home and Taliesin reveals he can lift the first three curse, but that he doesn't have the skill to reverse the curse of the Otherbody, and that he doubts anyone in Britain other then Merlin and Nimue do. Arthur and Merlin recalling Morgan was once Merlin's priestess send for her. She reveals that she can lift the curse but that it is against her Christian beliefs to do so. She finally agrees to lift the curse if Lord Derfel will become a Christian and swear allegiance to Bishop Sansum. He agrees to do so, Morgan then reveals that whatever binds Derfel to Nimue must be destroyed, which happens to be Derfel's hand. Lord Derfel allows his hand to be chopped off, Ceinwyn recovers and plans are made to sail for Dumnonia and join Sagramor.
Before they can set sail Nimue's army of the mad attack. Arthur's army escapes on the boats and Nimue sacrifices Merlin to Manawydan causing a storm to swamp all the boast except the one Arthur is in. With most of his soldiers lost at sea Arthur makes plans to flee Britain. Arriving in Camlann they discover Caddwg is caulking his boat and it will be the next day before they can leave. Reinforced by Sagramor's men they hold Mordered's army off until Caddwg is ready to leave. Arthur is severely wounded killing Mordred. Unable to take everyone Caddwg takes Arthur & Guinevere, Gwydre & Morwenna (their children), and Galahad away. Lord Derfel & Ceinwyn choose to stay behind with a dying Sagramor, and a dead Culhwch. Lord Derfel takes Excalibur and throws it into the sea. The fight ends and Meurig takes control of Dumnonia.