"Riddikulus" is a spell from the Harry Potter series, created by author J.K. Rowling. It is used to counter a Boggart, a shape-shifting creature that takes the form of a person's greatest fear. The spell transforms the Boggart into something humorous, allowing the caster to confront their fear in a less intimidating way. To cast "Riddikulus," a wizard or witch must focus on the fear and visualize a funny transformation. This spell is taught at Hogwarts School of Witchcraft and Wizardry during the Defence Against the Dark Arts classes, emphasizing the importance of humor in overcoming fear.