"Gwendy's Button Box" is a novella by Stephen King and Richard Chizmar, in which a mysterious stranger named Richard Farris provides a magical box to an average teenage girl. The girl uses the box to help herself lose weight, create gold coins, and solve other problems, but she is also plagued by fear of the box's ability to do evil, as well as fears of the box being discovered by others. The book takes place over the course of several years in the girl's life, during and after high school.
On a Reddit message board, Chizmar confirmed that Farris is an incarnation of recurring Stephen King villain Randall Flagg.