A single very significant moment, related to the fate of every single main character, and driving the entire plot, that happened before the beginning of the story. That event will probably be discussed, interpreted, viewed through flashbacks, and a The Reveal might show its true meaning later. In cases where the characters have any way of interfering with the past, this event will be their target. Since these these events are more exciting, and more detailed than your average background story, they are frequently used as the setting for a Prequel. See also Great Offscreen War.