To battle the giants released by Gaea, the Earth Mother, the Greek and Roman demigods will have to work together. They will then need to set sail for the old continent together in order to locate the Doors of Death. What are the Doors of Death exactly? A significant portion of the prophecy is still unknown. The Mark of Athena is another fantastic adventure by master storyteller Rick Riordan, with old friends and new allies joining forces, a magnificent ship, terrifying enemies, and some exotic environments.