The Once and Future King Book 4, Chapter 1

Several years later, Agravaine and Mordred talk about Guenever and Lancelot's affair. They both want to use it against the crown--Mordred because he hates his father, and Agravaine because he hates Lancelot. Mordred's reasons for revenge are more specific and craftier: Agravaine just enjoys being destructive. Agravaine says that they cannot start a conflict based on something personal: it has to be broadly political. Mordred reveals that Arthur tried to drown him when he was a baby, because he was the product of incest. Morgause taught Mordred, who grew up alone with her, to hate Arthur, but now that he is at court, Arthur is kind to Mordred, which confuses and angers him. Agravaine argues that bringing up the past will just make everyone hate Mordred. He continues that, were they to address Lancelot's infidelity directly, Arthur would be forced to investigate. His new form of justice requires that he try seriously to find the truth (rather than, for example, just killing the accuser.) No one has ever directly accused the Queen of the treason of cheating on her husband before, even though everyone knows about it. By splitting Arthur from his best knight, they could gain power at the castle.