What is the nightmare that descends upon the children in the novel To Kill a Mockingbird?

At the beginning of Chapter 15, Scout says, "A nightmare was upon us." (Lee 193) The nightmare Scout was referring to was the mob scene that takes place later on in the chapter. Harper Lee foreshadows this dangerous scene at the beginning of the chapter. Atticus goes by himself to sit outside of Tom Robinson's cell the night before the trial. Jem, Scout, and Dill follow Atticus and hide next to Tyndal's Hardware Store, and watch as the four cars arrive. The Old Sarum bunch gets out of their cars and surrounds Atticus. The mob plans on harming Tom Robinson. Scout runs into the group to greet Atticus and spots Walter Cunningham. After multiple attempts at "small talk," Scout gains Walter's attention. Walter realizes the precarious situation that Atticus is in, and tells the mob to leave. Scout unknowingly saves both her father and Tom Robinson from possible harm. The next day, Atticus teaches Scout and Jem a valuable lesson in mob mentality.