The climax of a story refers to the point of most drama or excitement in a story. This is usually the turning point of the narrative, and the moment most anticipated by readers.
In the case of "The Fall of the House of Usher," the climax of the story occurs when Roderick sees Lady Madeline burst through the door covered in blood. At this moment, the tension is extremely high, as the reader realizes that Lady Madeline was buried alive.