How did Mordred learn about the true fates of his foster parents in the novel, The Wicked Day?

Mordred carved a charm for Sula when he was a young boy living on the island. Sula treasured this simple gift and wore it every day of her life. When Gabran murdered Brude and Sula, he kept the charm as an amusing souvenir and carried it in his purse for years. His downfall was emptying his purse at a game of chance to illustrate he had no coins left, and Mordred recognized the charm. Gabran lost his life, Mordred knew the real story of his foster parents' death, and also knew the true killer, Morgause.