The Mark of Athena
The Mark Of Athena is the third book of the Heroes of Olympus series by Rick Riordan. In this book, the perspective of the story moves between Percy Jackson, Annabeth Chase, Leo Valdez, Jason Grace, and Piper McLean. All of the seven demigods appear in this book.
In the beginning, the Argo II is approaching Rome (Camp Jupiter) when suddenly, a statue pops into existence on the ship and almost knocks Annabeth overboard. The statue is Terminus, the Roman god of boundaries. He yells that he will not accept having weapons over the Pomerian Line (the camp boundary). After being irritated by a curious Leo and a charmspeaking Piper, Terminus tells them to "surrender, then leave". Annabeth sees Percy walking down in Camp Jupiter with Hazel and Frank as if they were best friends, and knew she was too close to give up. (She also notices that Percy is a praetor now and signs that New Rome had recently been attacked by an army of monsters.) She asks if it was against the rules to hover OVER New Rome, as technically it wouldn't be on Roman soil. Terminus agrees grudgingly to this, but tells them to not harm his city.