La Mort d'Arthur was a book written about the end of King Arthur's reign, recounting how he had been slain by Mordred at the battle of Camlan. The Fifth Doctor had read the book, and told a younger King Arthur about it to prove that contrary to Arthur's belief, Mordred had not been killed as a baby but rather lived on, and would one day kill him. The Doctor did not tell Arthur to try and avert this, but instead to focus on achieving as much in his reign before the grim event inevitably took place. (PROSE: The Creation of Camelot)
Behind the scenes[]
The book being referenced is usually referred to by its original Middle French title, Le Morte d'Arthur; for unknown reasons, the author of The Creation of Camelot decided to translate it into grammatically-correct Modern French, instead.