The Mark Of Athena (Heroes Of Olympus Book 3) is part of Rick Riordan’s The Heroes Of Olympus series. Taking off from where Son Of Neptune left off, the book tells the story of Greek and Roman demigods, in an alternating set of narratives.
The book begins with a prophecy recited before the Greek and Roman demigods, the significance of which is explained as the novel progresses. The Greek demigods plan to enter the Roman border, but are stopped by Terminus. Once a few of them reunite with the Greeks, they are told about a prophecy called The Mark of Athena, and that they should search for it to bring them both together. However, Leo’s mind is poisoned into attacking the Romans right away, which enrages them.
This follows the adventure of seven Greek demigods as they try to invade Rome by encountering several other demigods like Hercules and Ichthyocentaurs. Whether they find the Mark of Athena and whether all of them survive and unite, forms the rest of the gripping story.