Answer:
Learning that Magnus dies is a very important and unique plot device that Riordan uses. The lighthearted style of writing makes the tone feel humorous, but the idea of an agonizing death builds suspense. By learning that he dies, the reader questions how and why Magnus dies. The reader also wonders how he is writing to the reader if he really is dead. Not many writers write from the perspective of a dead character. Since death is the end of life, it is normally the end of a story. No so in "The Sword of Summer." In fact, Magnus' death sets the story up, explaining how he was accepted into Valhalla and so forth.
