The following version of this novel was used to create this study guide: Riordan, Rick. The Trials of Apollo: Book Four: The Tyrant's Tomb. Disney Hyperion, 2019. Hardcover.

As Apollo and Meg are transporting Jason’s body to Camp Jupiter, they are attacked by undead ghouls and forced off the road. They land in a forest where they are rescued by Lavinia and a group of fauns. Together, they carry the casket to a secret tunnel where a legionnaire, Hazel, fights off another pair of ghouls as they head into the city of New Rome. Once there, Hazel takes them to the senate house where Apollo explains that Jason died fighting Caligula, and the emperors are planning a second attack on Camp Jupiter in a few days. The soldiers, led by Frank and Reyna, prepare for the battle. Unfortunately, Apollo is injured by a ghoul and is slowly turning into an undead himself. Furthermore, Frank tells Apollo that the goddess, Hela, linked his life force to a piece of firewood. If one burns, so does the other.

After confirming a prophecy with Ella, the group learns that they need the breath of a soundless god for a ritual to summon a god from Olympus. Apollo, Hazel, Meg, and Lavinia find the tomb of the undead king Tarquin and learn that the soundless god is on Sutro Tower. Unfortunately, the meeting quickens Apollo’s curse. With very little time left, Apollo, Meg, and Reyna head to Sutro Tower, knowing that the emperors are close to the camp with weapons of mass destruction. They find the soundless god on top of the tower, and learn that he is holding the voice of the Sibyl that Apollo once cursed in a jar. Apollo, who is extremely remorseful of his past mistakes, begs the god to help his friends, even if it means his life. The Sibyl tells Harpocrates that she saw this future, and they need to let go. Reyna retrieves the jar as Caligula’s yachts reach Camp Jupiter.

Luckily, Caligula allows Commodus to send the army in first instead of using their weapons, giving Reyna, Apollo, and Meg time to reach them. Unfortunately, Reyna is injured in a ghoul attack, leaving Apollo and Meg to continue alone. They fight as many enemy troops as they can before finding Frank. He tells Apollo to perform the ritual as Hazel and Meg help fight Tarquin in New Rome. Apollo decides to summon the roman incarnation of his sister, Diana. But when he completes the ritual, nothing happens, and Apollo returns to Frank, disappointed.

As Caligula and Commodus prepare to launch their weapons, Frank challenges them to a duel. Apollo joins, but Frank quietly tells Apollo that he must run when Frank says that time is up. The four start fighting, and Apollo is overwhelmed by Commodus. However, he is able to gain a small advantage by stabbing Commodus while the emperor rallies the crowd. Both he and Frank retreat into the tunnel, where Frank ignites his firewood and blows himself and Caligula up. Commodus survives, but Apollo screams in rage, killing him. Meanwhile Laivinia uses water sprites to turn the yacht’s weapons against them, and the army flees.

Apollo returns to New Rome where he, Meg, and Hazel find Tarquin in the bookstore looking for the books of prophecy. They fight him, but he is finally killed when Diana arrives, having wiped out his army on the way. She apologizes to Apollo for being late, but says that even Jupiter (Zeus) could not ignore a summons from Temple Hill. Once she leaves, Reyna is inducted into the Hunters of Diana (Artemis) and leaves with Thalia and the others. Hazel takes Reyna’s place, and Lavinia is promoted to lead Hazel’s former cohort. Frank is found alive, and Apollo believes that he survived because he accepted his fate rather than running away from them. The members of Camp Jupiter then gift Apollo with his old bow, which he is thrilled to be able to use as it means his godly power is returning. They also give Meg a pouch of ancient seeds that she can grow with her own demi-god powers and send them on their final quest; to save the oracle Delphi by defeating Nero and Python.