Twilight wakes up one day to find that things are different. Everypony is gone, and there's a persistent sheet of fog dominating all of Ponyville. But that's the least of her worries. It only escalates from there.
(Featured on EqD) A dark and tragic event occurred some years ago in Ponyville, and it involved an equally dark and dysfunctional family. They are still discussed among us to this day.