The Once and Future King Book 3, Chapter 36
The court has changed. Most of the best knights died in the quest for the Grail, and all the youthful excitement of Arthur's Table has died away. Everyone is most interested now in Guenever's infidelity. She has never been easy to like, and now she has become unpopular. When a knight tries to poison Gawaine at dinner in revenge for Pellinore's murder, another knight dies by mistake. Because Guenever served the food, she is accused of the murder. She needs someone to fight for her, (that is the way guilt or innocence is decided) and she begs Bors (who never liked her) to do it. She cannot find anyone else willing, and Arthur, though he believes she is innocent, feels it is his duty to allow a trial.