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The Staff of Serapis is written by Rick Riordan and the second crossover between The Kane Chronicles and Percy Jackson and the Olympians. It is the sequel to The Son of Sobek where Annabeth Chase and Sadie Kane meet. It came out on April 8th, 2014 in the back of the paperback of The Mark of Athena. The release date of the e-book and audiobook was on May 20th, 2014. The cover was released April 3rd, 2014 in a live interview with Rick Riordan.
In this adventure, Annabeth encounters more oddities in the subway than usual, including a two-headed monster and a younger blond girl who reminds her a little of herself... This is the story that fans have asked for, in which Annabeth Chase teams up with Sadie Kane. The demigod daughter of Athena and the young magician from Brooklyn House take on a larger-than-life foe from the ancient world. Perhaps even more disturbing than the power-hungry god they encounter is the revelation that he is being controlled by someone that is all too familiar to Sadie. The audio narration was recorded by Rick Riordan himself. As a special bonus, the first chapter of The Blood of Olympus, the final book in The Heroes of Olympus series, is included. 
After presumably failing an interview, Annabeth is going to the subway to go to Percy's apartment to cheer her up. Suddenly she sees a monster with a wolf and a lion head sticking out of a cone-shaped shell. The monster glares at her and then turns away as if it had business elsewhere. Annabeth decides to follow the monster, as they normally do not leave demigods, like her, alone. After this, she realizes that she doesn't have her knife so she starts making the heads argue with each other. A train pulls in and Sadie casts a few spells on the monster.
The monster starts running away and Annabeth and Sadie introduce themselves. Sadie also has a dog that just appeared in her library that she named "Fido". She asks Annabeth to come to walk her dog with her, and they exchange stories, both of them finding out about the other's world. Annabeth also remembers Percy recalling fighting a giant crocodile with Carter. Sadie however, didn't know that and said she'd confront Carter later. Fido then breaks loose and both Sadie and Annabeth are blown backward.
Annabeth picks up Sadie's wand and it turns into a dagger. Sadie tells her to keep it. Sadie then realizes that Fido was heading straight for the "storm". Annabeth doesn't know what she meant until Sadie rubbed something pink on her eyes and her eyes were able to see the Duat. They then head after Fido, but they come too late because he merges with the monster. They then realize that they see a man on top of the building.
They start figuring out this monster and this man (a god) were both Egyptian and Greek from the time when Ptolemy was the pharaoh. The structure of the building is also changing. Sadie says that they'll split up and that Annabeth could deal with the god that they couldn't remember the name of with her staff creating a forcefield around her.
He keeps ignoring her until she figures the god's name and yells "SERAPIS". Once she gets Serapis' attention shestarts talking to him trying to give Sadie enough time "to do whatever Sadie was going to do". She saw that Sadie was drawing chalk circles, and once they were complete she said the spell "tyet" and then said the "ha-di" spell. However, Serapis just waved it away like it were a fly. When that doesn't work she yells "Suh Fah!" and the entire wall collapsed on him.
They get off the building and see Serapis again. Annabeth tells Sadie what she learned from talking to him. He was building a new lighthouse of Alexandria, and he wanted all the gods to come to it so he could destroy them, etc. They then form a new plan to get rid of Serapis because he emerges from the rubble. Annabeth teaches Sadie some Greek and she starts chanting Egyptian and Greek to confuse Serapis.
Then Annabeth, with the invisibility hat on, destroys the dog head, representing the future on Serapis's staff (there are three heads on the staff). Serapis is then vanquished, and Sadie tells Annabeth about her speculations that Setne was the one who released the Son of Sobek and Serapis. Sadie and Annabeth exchange phone numbers and part ways.
- In the e-book, a sneak peek of the first chapter of The Blood of Olympus is included.
- Author Rick Riordan reads for the audiobook.