The Battle of the Labyrinth is a 2008 fantasy-adventure novel based on Greek mythology; it is the fourth novel in the Percy Jackson & the Olympians series by Rick Riordan. Fictional demigod Percy Jackson who is fifteen years old by the end of the book tries to stop Luke Castellan and his army from invading Camp Half-Blood by navigating Daedalus's labyrinth. It was released on May 6, 2008 in the US and Canada. It was received positively overall, with the novel being a #1 The New York Times children's series best seller and a #1 chapter book bestsellerDuring an assignment to rescue Nico and Bianca di Angelo, Annabeth Chase and Artemis, the goddess of the hunt, go missing. Percy and his friends must look for Artemis before the winter solstice meeting of the Olympian council, when the goddess's influence could change an important vote on the war with the Titans. Percy joins Thalia, Zoë Nightshade, Bianca di Angelo, and Grover on their dangerous quest. They also are searching for a rare monster that Artemis was trying to hunt down when she went missing; it is so strong that it can obliterate Olympus. Percy is called by a hippocampus (mythological sea-horse) to rescue a marine cow-like creature called the Ophiotaurus, whom Percy names Bessie, not knowing it is male