In the book "Le Morte d'Arthur", who exposes Queen Guinevere's affair with Lancelot?

"Le Morte d'Arthur" ("The Death of Arthur") is a 15th-century Middle English prose reworking by Sir Thomas Malory. It is tales about the legendary King Arthur, Guinevere, Lancelot, Merlin and the Knights of the Round Table, along with their respective folklore. To give a complete story of Arthur from his conception to his death, Malory compiled, rearranged, interpreted and modified material from various French and English sources. "Le Morte d'Arthur" is considered one of the best-known works of Arthurian literature. Many authors who have written about King Arthur since the 19th-century revival of the legend have used Malory as their principal source.

In Malory's medieval Arthurian work, Mordred is knighted by Arthur and joins the fellowship of the Round Table. In this narrative, he eventually becomes the main actor in Arthur's downfall: he helps his half-brother Agravain to expose the affair between Guinevere and Lancelot, and then takes advantage of the resulting civil war to make himself the high king of Britain.

The prominent story arc concerning the queen's tragic love affair with her husband's chief knight and trusted friend, Lancelot, indirectly causes the death of Arthur and the downfall of Arthur's kingdom.