An adventure-fantasy pentalogy centered on a reimagined version of the Greek god Apollo, The Trials of Apollo series has been called “the capstone on Percy Jackson’s world” by Riordan. The series is the culmination of the world Riordan introduced with his Percy Jackson books in 2005. In the Percy Jackson world, Greek gods are real, and demigods, their offspring with mortals, live on earth. With The Percy Jackson series, Riordan introduced many contemporary readers to the elements of Greek mythology. In The Trials of Apollo books, the Percy Jackson world features gods, monsters, and demons from the Greek and Roman pantheon and figures from ancient Greco-Roman history.

The action in The Trials of Apollo follows events in The Heroes of Olympus series, the second to feature the Percy Jackson world. Zeus, the chief of gods, blames his son Apollo for playing a part in events that lead the angry earth-mother Gaia to open the doors to the underworld, resurrecting all manners of monsters and dead villains and paralyzing the Olympian gods with pain. At the beginning of The Hidden Oracle, the first book in The Trials of Apollo series, Zeus transforms Apollo into 16-year-old mortal Lester Papadopoulos as a punishment and drops him from the heights of Mount Olympus into a garbage dump in New York.