Sarabeth was a boarder who was taken into the home of Evan Ross' Aunt Kathryn nearly 30 years prior to the episode's events. However, when Kathryn discovered that Sarabeth was actually a witch practicing dark magic, Kathryn attempted to oust her, but Sarabeth's power was too strong. During the scuffle, Sarabeth was consumed by monster blood, and after that, Kathryn placed the substance in a jar and locked it away.
Evan and his best friend Andy (short for Andrea) would find the monster blood in the episode, and in the process, the evil Sarabeth was freed; appearing as a black cat before morphing into her real form. Sarabeth later summoned the monster blood to consume Kathryn, Evan, and Andy, only to be startled by Evan's dog, Trigger, who became enlarged due to the monster blood's effects. While attempting to fend off Trigger, Sarabeth backed into the monster blood, and she was consumed and destroyed for good.
In the book, it is stated that Sarabeth cast a spell that turned Kathryn into her slave. She also made her deaf and refused to teach her sign language and lip reading, so she could control her better. This part was left off the TV version, though Sarabeth's fate was the same in both versions.