In The Son of Neptune Hylla has become the Queen of the Amazons after her confrontation with Percy and Annabeth. It is also revealed that Reyna is her sister, and they have both become important leaders since Circe's Island. When Hazel, Percy and Frank arrive at the Amazon base, Hylla has Percy as well as Frank locked up and holds a conversation with Hazel and Kinzie. Hylla also says that she knows there are both Roman and Greek demigods, but treats it as if it as nothing. She tells Hazel that Otrera, the first Amazon queen was reborn thanks to the Doors of Death being open and she is gaining support from some of the Amazons. Otrera plans to challenge Hylla in a battle to the death for control of the Amazons as anyone can challenge the queen for their position. While the challenge is a battle to the death, Otrera can't die and would reform everyday until she won, making Hylla worry that she would be killed and the Amazons would follow Otrera into battle against Camp Jupiter. Hylla would like nothing more than to help as her sister, Reyna, as she is praetor for the camp.
Hylla, when learning that the three are on a quest to free Thanatos, she tells Hazel that she can't set her friends free as it would look like she was weak, but if they managed to escape somehow on accident and defeat the Amazons that had betrayed her and eere now loyal to Otrera, they could escape on Arion, a horse none of the Amazons could tame, but took a liking to Hazel after she fed him a gold brick she summoned from the ground. She tells Hazel that she will hold out against Otrera as long as she can until they finish their quest and would help her sister if she survived. During the battle at Camp Jupiter, when the forces of the two armies was almost even, the Amazons appear blowing their horns ready for battle, much to Polybotes delight until he noticed Hylla had managed to kill Otrera two nights in a row. Hylla led her warriors into battle and helped push the Giant Army into a retreat. During the celebration after the battle, she spends time with her sister. The following day during council meeting, she sided with Percy about letting the Greeks land when he made a comment that no one expected. That the Romans and Amazons to work together a week ago, saying, "He's got a point!" Reyna thanks Percy Jackson, as she had told him to ask Hylla to help them, and he succeeded.