18 of Rick Riordan's The Lightning Thief, first book of the "Percy Jackson and the Olympians" series, is a struggle for power. Often in myths, two adversaries battle each other for the sake of control due to vanity or out of jealousy. The two adversaries can be "two supernatural forces, a supernatural force and a mortal, or two members of a single family" (Kyrene School District, "The Key Elements of Mythology"). In Chapter 18, Percy, Annabell, and Grover must face the monster Cerberus in order to enter the Underworld. Cerberus is described by Percy as a giant three-headed Rottweiler, and Cerberus's job is to prevent anyone who is not dead from entering the Underworld. Hence, Percy, Grover, and Annabeth engage in a power struggle with Cerberus for control over access to the Underworld. Percy first attempts to distract Cerberus by throwing him a stick to fetch. However, Annabeth proves to be the one who is more skilled in dog obedience training and tempts Cerberus with a red rubber ball. Wanting the rubber ball, Cerberus immediately obeys her command to "Sit!" and tosses the ball back to Annabeth, simultaneously allowing Percy and Grover to pass around him into the Underworld, followed soon by Annabeth. Hence, Percy, Grover, and Annabeth were able to win in their power struggle against Cerberus over control of entrance to the Underworld by taming Cerberus.

This same power struggle also relates to a second key element of mythology referred to as "tension between opposing forces in the universe" (Kyrene School District). Opposing forces can be considered the forces of good and evil, and a hero can be seen as embodying the force of good, whereas a monster can be seen as embodying the force of evil. Hence, when the kids undertake subduing the monster, they are also undertaking subduing the forces of evil, showing that they are engaged in tensions that are not just between the kids and the monster but between the force of good and the force of evil. Yet, based on Cerberus's response to the ball and the attention Annabeth gives him, we see that Cerberus is not truly the evil one; instead, he is surrounded by evil that has influenced him. In reality, Cerberus is just like any dog--able to respond to affection with love and loyalty. We especially see Cerberus's true nature when Percy ends the chapter with the following observation:

I pretended not to see Annabeth wipe a tear from her cheek as she listened to the mournful keening of Cerberus in the distance, longing for his new friend. (p. 313)

Other key elements of mythology to consider with respect to Chapter 18 concern the use of supernatural characters with human characteristics and the ways in which myths are used to explain the natural world.