This article is about the coin. You may be looking for the book.
The Mark of Athena was a coin that the goddess Athena gave to a select few of her demigod offspring to avenge her against the Romans for stealing her sacred statue. The coin leads its possessor to the Athena Parthenos in Rome, where Athena's rival, Arachne, was guarding the statue in a chamber.
Every few generations, if Athena considers one of her children worthy, she gives them a silver drachma. The image of Athena's sacred animal, an owl, appears and "guides" the possessor to the statue.
It is shown that Athena had given Annabeth Chase the coin prior to the events of the book. She had told Annabeth to follow it and "avenge her," or else she was no child of hers. The coin ultimately leads Annabeth to the Athena Parthenos, where she faces Arachne, the greatest fear of the children of Athena.