This five-book middle-grade fantasy series follows the Greek god Apollo, who has been cast down from Olympus in human form as punishment and must reclaim his divine status by completing a series of dangerous quests — accompanied by demigod allies from the Percy Jackson universe. The series continues the author's mythology-based universe with humor and adventure while dealing with darker themes than earlier installments, including mortality and the consequences of past arrogance.
The god Apollo is cast down to earth by Zeus and must survive as a pimpled, powerless sixteen-year-old mortal named Lester, relying on a demigod named Meg to help him find his way back to Olympus and discover why the Oracles have gone silent. The first Trials of Apollo novel is funnier and more self-aware than previous series, with Apollo's self-centered narration providing fresh comedic perspective on the familiar world.
Apollo and Meg travel to Indianapolis to free the second Oracle while being pursued by the Triumvirate of evil emperors, meeting Leo Valdez and Calypso along the way and confronting an ancient evil beneath the city. The second Trials of Apollo novel deepens the series' broader conflict and gives Leo and Calypso strong showcase moments alongside the central characters.
Apollo and Meg travel to Southern California to confront the emperor Caligula and free the third Oracle, with help from Piper and Jason, in the Trials of Apollo's darkest entry—one that carries a significant and emotionally devastating loss of a major series character. Riordan makes it clear that the series is willing to pay real costs.
Apollo and Meg travel to Camp Jupiter, the Roman demigod camp, to free the fourth Oracle while mourning the loss they suffered in the previous book and facing a new threat from the undead emperor Caligula alongside the third emperor Commodus. A gripping penultimate installment that deepens the emotional stakes and sets up the series finale.
The Trials of Apollo concludes as Apollo and Meg return to New York for a final confrontation with the emperor Nero and the shadowy mastermind behind the Triumvirate's plans, requiring Apollo to face the deepest test of his transformation from a selfish god to something genuinely heroic. A satisfying conclusion to the five-book series and a meaningful close to Riordan's extended Camp Half-Blood universe.