The Once and Future King Book 4, Chapter 13
Inside Lancelot's castle, no one knows why Arthur and the others have left for England so quickly. Suddenly Lancelot comes in, holding a letter, and tells his soldiers that they must go to England immediately. He tells them about Mordred's plans. The letter turns out to be from Gawaine, who is dead. When he was dying, he wrote to Lancelot to forgive him for killing his brothers, and to tell him to pray for him, and to ask him to help Arthur against Mordred--his last brother.