The book ends with a mature (read: old) King Arthur hanging out in his royal pavilion on the battlefield the night before he's due to engage with Mordred in The Final Battle. He's doing some heavy-duty thinking about all of the lessons he's been trying to learn about Might and Right. He thinks way back to his childhood, when he had Merlyn for a tutor and he was just known as Wart.
Arthur dwells on how he's tried his best to channel Might into serving Right (founding the Round Table), and then trying to control Might further by setting up a system of laws that were supposed to bind it. But that didn't work, because in the end, the crowds won out and demanded another bloody fight.
He calls in a page, who turns out to be Thomas Malory, and tells him his entire story, entrusting his ideals to him—ideals he compares to a candle that he's been protecting and is now handing over to the younger generation to keep safe. After Thomas departs, Arthur despairs at how he's tried his best, but it hasn't been good enough. He looks forward to a time when humans can ignore the imaginary boundaries on the map that cause war.
In a sudden flash-forward, we learn that Arthur dies (or is taken off to the magical island of Avilion, to return again), Mordred dies, and Lancelot and Guenever take up the religious life.
Arthur wakes up the next morning to meet his fate on the battlefield, but this is only the beginning.