Excalibur: A Novel of Arthur is a historical fiction novel by English writer Bernard Cornwell, first published in the UK in 1997. It is the third and final book in The Warlord Chronicles series, following The Winter King and Enemy of God. The trilogy tells the legend of King Arthur through the eyes of his follower Derfel Cadarn.

Embittered by multiple betrayals, Arthur once again assumes responsibility for the defense of free Britain, and as the Saxons threaten imminent conquest he assembles the resistance in a valiant last effort to defeat them. Merlin attempts to summon the gods to earth, but Arthur and Derfel disrupt the ceremony, earning them Nimue's wrath. Though Arthur manages to halt the Saxon advance and repair his broken marriage, his dream of a simple life proves forever out of reach as dark forces renew their commitment to undo, once and for all, his vision of a peaceful Britain.

Excalibur begins shortly after the end of the second novel, Enemy of God. Arthur has taken full control of Dumnonia, Guinevere is imprisoned, and Mordred, while nominally still king, has been stripped of his power as punishment for his treachery. Lancelot has joined the Saxons, while the Saxon kings Aelle and Cerdic have put aside their differences in an attempt to finally subjugate the Britons. In the wake of Guinevere's infidelity, Arthur has become a bitter man, and Derfel notices his friend is cold and distant. Arthur has persuaded Dumnonia and Powys to unite against the Saxons, and has secured the support of several Irish kingdoms by remarrying to the young Irish princess Argante, but Meurig, king of Gwent, will not send his formidable army unless he is given the throne of Dumnonia. Arthur refuses his demand, having long ago sworn an oath to preserve Mordred as the rightful king, and instead sends Derfel to try to persuade Aelle, Derfel's father, to turn against Cerdic. Though the Saxons have been murdering all British emissaries, Derfel travels to meet the Saxon kings in Lloegyr. Aelle welcomes his son but Cerdic demands Derfel's death, which Aelle settles by insisting that Derfel fight a Saxon champion in single combat. Derfel defeats Cerdic's champion Liofa but spares his life, and then parlays with Aelle, who is not fooled by Arthur's false promises and reasserts his intent to invade Dumnonia.

Meanwhile, Merlin prepares a pagan rite at Mai Dun which he believes will bring the Old Gods back to Britain by gathering together the Thirteen Treasures of Britain, including Arthur's sword Excalibur, which is revealed to be the Sword of Rhydderch. Derfel is inspired when he sees the beautiful silvery skin of Olwen. He also meets Gawain, a naïve young prince of Broceliande and Arthur's nephew, whom Merlin has convinced to remained celibate under the pretense that he is destined to be the saviour of Britain. As an enormous balefire begins to burn atop Mai Dun, Arthur and Derfel suddenly realise that Merlin and Nimue have kidnapped Arthur's young son Gwydre, intending to sacrifice him as part of the ritual. They climb the burning hill and rescue Gwydre, but find that Merlin has already killed Gawain. Arthur's interruption of the ceremony earns him the hatred of many pagans in Britain, particularly Nimue, who blames him and Derfel for preventing the prophesied return of the gods.

After a long winter of anticipation, the Saxons finally begin their invasion with a surprise attack during the pagan holiday of Beltain. Derfel scrambles to bring his small command to Corinium, where Arthur's army plans to meet the Saxon assault. Followed by refugees fleeing Dumnonia, Derfel frees Guinevere from her prison atop Ynys Wydryn and attempts to bring her and the other families to safety at Glevum, but they stumble into a Saxon regiment and are trapped on a hilltop near Aquae Sulis. Unable to escape to aid Arthur, the greatly outnumbered Britons dig in for a long siege. Derfel is impressed by Guinevere's efforts as she uses several tricks to hamper the enemy forces, winning them much needed time and confidence. Eventually, Arthur's horsemen arrive at Mount Baddon along with Sagramor and King Cuneglas of Powys and manage to win a victory over the Saxons. Arthur also succeeds in convincing Gwent to fight by appealing to Tewdric, Meurig's father, who had long ago abdicated to become a monk, with the price of Tewdric's support being that Arthur fight in the name of the Christian god. Tewdric briefly reclaims his throne from his son and leads Gwent's thousand spears in battle against the Saxons.

During a respite in the fighting, Cuneglas is drawn into single combat with Liofa, Cerdic's champion, but is killed, costing Arthur his staunchest ally and Derfel a man he considered a brother. Despite assistance from Tewdric, the Saxons still outnumber the British and, despite their heavy losses, they are likely to win the battle. However, just as the British brace for a third assault, Culhwch and Irish king Oengus mac Airem arrive with fresh spearmen. Merlin spurs a horse to charge the Saxon line with Gawain's corpse as the rider, terrifying the Saxons into believing a ghoul is attacking them. The Saxons are overwhelmed and routed, and Cerdic flees with his army in ruins while Aelle's remaining forces are surrounded. Aelle asks his son to give him a warrior's death and Derfel obliges, despite fearing his mother's prophecy that he would kill his father. He buries Aelle with honor according to the Saxon rites and spares the lives of his men. Lancelot is caught hiding in a clump of debris on a riverbank, and Derfel forces him to accept a challenge to single combat; despite his cowardice, Lancelot proves to be a competent swordsman, but Derfel defeats him anyway, and has him hanged from a tree.

The power of the Saxons is broken. Cerdic, now greatly weakened, is forced to retreat, and Sagramor reconquers territory in western Lloegyr. Tewdric is venerated as a saint for his actions at Mynydd Baddon and the Christians credit him with the victory. Tewdric once again renounces his throne, allowing Meurig to return to power. Lancelot goes down in legend as the greatest hero of Arthur's retinue, when in truth he was a coward, a rapist, and a traitor.

Guinevere explains to Derfel that Arthur's price for gaining Gwent's support at Mynydd Baddon was giving up his power and forever abstaining from British politics. As this is what Arthur had always wanted, he agreed, and now leaves Derfel and Sagramor as the leaders of Dumnonia's army: though Mordred is still king, Derfel will command the spears in Dumnonia itself, while Sagramor will watch the border with the Saxons. Arthur and Guinevere have also agreed to move to Silurian Isca, where Meurig can keep an eye on him. Although Guinevere admits that it was not what she wanted for herself and Arthur, she still loves Arthur and wants to be with him. Arthur avoids insulting the Irish by proposing that his recent bride, Oengus mac Airem's daughter Argante, be remarried to Mordred. Arthur and Guinevere rebuild their marriage in Isca, hoping to finally live a simple, peaceful life with their son Gwydre, though they are accompanied by a warband as Arthur still has enemies.