The Once and Future King Book 3, Chapter 31
For a long time no one hears from Lancelot. There are rumors of him being defeated in battle, going mad, or being killed. Guenever is disturbed, and often almost says something to bring her affair with Lancelot out in the open. Mordred and Agravaine, who dislike Arthur, Guenever and Lancelot--indeed, they resent the entire idea of the Round Table--wait for her to slip. Finally, Lancelot comes home, nearly dead from exhaustion. He has been torturing himself, trying to become pure. He tells his friends that Galahad, Percivale and Bors have found the Holy Grail and gone to Babylon. Bors will return, but the others--and the Grail--will never be seen again.