Answer:The reward for beating the Traitor Lord is access to the White Lady. Interacting with her gives you the first half of the Kingsoul charm. You can get the second half of the charm from the White Palace, which grants you access to the Birthplace in the Abyss, transforming the charm into the Void Heart. The Void Heart unlocks new endings and allows access into the Pantheon of Hallownest. You also receive the Execution achievement after your encounter with the Traitor Lord.