In antiquity, the Argonaut Periclymenus, received from his grandfather, Poseidon, the gift to change into any animal at will. Centuries later, Seneca Gracchus, one of Pericylmenus’ shapeshifter descendants who served in a legion of the Roman Empire, participated to the Battle of Carrhae against the Parthians on the eastern border of the Empire. This battle was a huge disaster for the Romans and Gracchus was captured with 10,000 legionnaires, and put to work in Asia, by the Parthians, until the attack of Chinese people, who took the prisoners to their lands. The Romans eventually put down roots, mixed with the local population, and built a new hometown, Li-Jien (Legion), in which the descendants of Seneca Gracchus, known in China as Sung Guo, and their gift prospered for millennia.
Sung Guo was also known for having fought twelve Roman dragons and sixteen Chinese dragons in the western deserts of China.
In the nineteenth century, the Chinese descendants of Graccus and Periclymenus immigrated to Canada, and in the early twentieth century, a member of the family, Shen Lun, was invited to join Camp Jupiter as a legacy of Neptune. But because of his Asian heritage, his kinship with Neptune and his unusual gift, he was blamed without evidence for the earthquake of San Francisco in 1906. His granddaughter, Emily Zhang, later gave birth to the demigod, Frank Zhang, who combines the metamorphosis gift of his family and the war powers conveyed by his father, the Roman god of war, Mars.
He does not formally appear in the book, but is mentioned by Emily Zhang in the rainbow image Iris had made. Emily said he had been able to beat the dragons because he had been the strongest dragon of them all. This confuses Frank during his childhood as he could not understand how his ancestor could be a dragon. This serves as a clue to what his family's gift is, and Frank understands Sung Guo was able transform into a dragon. He is also the reason the Lastrygonians wanted to kill Frank, as they have been persuaded that by killing the last Zhang they will inherit his ancestor's powers.
- The last name "Gracchus" was most likely based on Tiberius and Gaius Gracchus, two tribunes of the Plebs who lived in Ancient Rome.