What did frank retrieve from the river after the two gorgons died the son of neptune?

In "The Son of Neptune" by Rick Riordan, Frank Zhang retrieves a lost, ancient artifact from the river after defeating the two Gorgons. This artifact is a piece of his past, specifically a shield that belonged to his ancestor, which symbolizes his heritage and connection to his family's history. This moment is significant as it helps Frank embrace his identity and powers as a demigod.