In the eighth book in Malory’s Le Morte D’Arthur, Arthur’s son Mordred has come to court, and pressures King Arthur to gather evidence about Lancelot’s and Guinevere’s affair. Arthur refuses, and Mordred sets a trap for them. Lancelot fights his way out, but Guinevere stays behind.
King Arthur feels he has no choice but to condemn her to death. But he leaves the scene of her would-be execution under-guarded, with the hope that Lancelot saves her. He does, but he kills several knights of the Round Table. In the fray, he accidentally kills one of his closest friends, Sir Gareth, the youngest of the Orkney brothers.
Lancelot and Guinevere escape to France, where Lancelot’s father has a castle. King Arthur is pressured by the surviving Orkney brothers and by Mordred to lay seige to the castle. He does so, and Mordred stays behind in England, plotting to take over the kingdom. When Arthur discovers Mordred’s betrayal, he returns to England and fights in a final battle against Mordred. Arthur and Mordred mortally wound each other. As Arthur is dying, he throws the sword Excalibur back to the lake guarding Avalon, and the Lady of the Lake reaches from within the water’s depths and takes the sword.
With no one left to rule, Arthur’s kingdom falls into chaos.
Guinevere and Lancelot blame themselves for the fate of Arthur and the other knights who died during the battle, and they part. Guinevere spends the rest of her life in a nunnery, Lancelot lives out the rest of his days in a monastery.