An event that took place before the start of the series that is related to many of the events that occur in-series.
Long ago in the kingdom of Troperia, a great disaster swept through the land. Many lives were lost and, to this day, no one knows what caused the event that has come to be known only as The Cataclysm. Some say that the members of an ancient society with mysterious, unknown goals were behimd it. Others say that the powers that govern the people are hiding something. No one is completely sure how or why The Cataclysm came to pass, but (almost) all can agree: the world would be better off if The Cataclysm never came to repeat itself... ...A typical use of the The Cataclysm trope. Often used in fantasy works, The Cataclysm is a trope that can easily set up the events of a story, or possibly the entire setting itself. The Cataclysm can be used in many different ways. It can sometimes provide a touch of mystery to a story, only being mentioned every now and then and possibly kept unexplained until much later in the story (if it's ever explained at all.) If done well, The Cataclysm in question will be deeply tied in with the events of she story and explained in detail. If done poorly, it will bew used as an easy way of filling any given Plot Hole, (ie: Where did that Humongous Mecha come from? It was revealed in The Cataclysm.) Contrast First Episode Spoiler.
Examples:Anime and Manga