Cataclysm Backstory

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 behind 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 Cataclysm Backstory trope.

Often used in fantasy works, Cataclysm Backstory is a trope that can easily set up the events of a story, or possibly the entire setting itself. It's very common in dystopia works and often used in stories that take place After the End. Cataclysm Backstory can be used in many different ways. It can sometimes provide a touch of mystery to any work, 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, Cataclysm Backstory in question will be deeply tied in with the events of the plot and explained in detail. If done poorly, it will be used as an easy way of filling any given Plot Hole, (ie: Where did that Humongous Mecha come from? It was revealed during the Cataclysm.)

Contrast First Episode Spoiler. Compare Great Offscreen War.

Anime and Manga

 * Guilty Crown has Lost Chrsitmas, a well-done case where the Apocalypse Virus was released. The first two minutes after the opening of episode one are used to explain how Lost Christmas caused Japan to clamor for help from outside governments in order to keep the virus under control, sacrificing their independence in exchange for stability and setting up the events of the main series ten years later. It goes a lot deeper than that, though.
 * Demon City Shinjuku. Rebi Ra kills his opponent Genichirou and causes an earthquake, devastating the Shinjuku area of Tokyo and leaving it a demon-haunted ruin. Ten years later Genichirou's son must enter Shinjuku and stop Rebi Ra from performing a ritual that will allow demons to conquer the Earth.
 * The Big O. Forty years earlier an unknown disaster wiped out human civilization. Only those within the main city and its surrounding landscape remain, but none can remember what happened on that day.
 * The Hydrus Beta supernova shockwave that very nearly wiped out humanity 200 years ago in Stellvia of the Universe.
 * In Akira, the Fall of Tokyo, triggered by the awakening of the title character's incredible psionic abilities.

Literature

 * The Seven Realms Series gives us The Breaking. At first kept very ambiguous, its eventually explained (in much detail) that many millennia ago, The Demon King kidnapped princess Hanalea and subsequently performed an act of dark magic that caused a series of natural disasters to sweep the whole of the Seven Realms. Unluckily for him, Hanalea was a Badass Princess that eventually brought him down.  Anyway, fast forward ten thousand years, and street rat Han Alister gets ahold of the Demon King's amulet.
 * The Sixty Minute War in Mortal Engines.
 * The Hunger Games: "A war" is about as specific as it gets to explain why the districts are divided as they are, and why the Games even exist.
 * Divergent: Of the "no one knows" variety, something has caused Chicago to isolate itself from the rest of the world, turning the society into the five virtue-based factions in the story.
 * The Doom of Valyria in A Song of Ice and Fire.
 * The Witcher series had the Conjunction of the Spheres, which occurred several centuries prior to the saga and brought hordes of chtonic monsters into the world, where they thrived on a healthy human diet. The eponymous witchers (superhuman monster hunters) were invented to protect the local humanity from these monsters, though they've been so efficient over the centuries that their existence is almost obsolete by the time the books takes place, as there are very few monsters for them left to hunt.

Radio

 * "The Withering" in Nebulous.

Tabletop Games

 * Greyhawk has the Twin Cataclysms: the Invoked Devastation and the Rain of Colorless Fire.
 * The Day of Mourning that played a big part in ending the Last War in Eberron. No one knows what caused it or which of the Five Nations was responsible, but we know it wiped Cyre off the map and rendered it an inhospitable wasteland known as the Mournland.
 * Dragonlance has the Cataclysm, which went down when the Kingpriest of Istar demanded to be given godlike powers to eradicate evil from the world (more specifically, to genocide the ogre races and relocate the dwarves and kender). The gods were not pleased, and punished him by wiping the city of Istar off the face of Krynn, creating what has since been called the Blood Sea of Istar due to the thick red soil churned up by the sea creating a blood-like appearance; as well as splitting apart the Kingdom of Ergoth, messing life up for several cities (one inland city found itself becoming a coastal city because of a new inland sea, and a port city found itself becoming an inland city surrounded by plains), and outright destroying several more cities including Xak Tsarosk.

Video Games

 * The Black Beast rampaging all over the world in Blaz Blue.
 * War Craft's Sundering is this, although the more recent event that is actually called the Cataclysm is not.
 * White Knight Chronicles gives us, well, the Cataclysm. It happened about seventeen years before the start of the story and it's only mentioned a few times in the entire series, but it's the basic explanation for how the Yshrenian Knights  were revealed. No real explanation is given, but it's heavily implied to be because it was simply time for Emperor Madoras to rear his ugly head again, and for him to be defeated.
 * The Collapse, a oft-mentioned but never-explained cataclysm that separates The Longest Journey and Dreamfall.
 * Whatever destroyed the Prothean civilization in Mass Effect 1.
 * Hinted at in The Legend of Zelda Skyward Sword, and, by extension, the rest of the Zelda series to which it is a prequel. The cataclysm in question was invasion by demons, and before it, there was a civilization of sapient robots with technology to put modern Earth to shame. After it, the setting Medieval European Fantasy with a bit of Lost Technology scattered here and there, which then becomes the status quo for thousands of years.
 * Fallout takes place in the aftermath of the Great War, a nuclear apocalypse that created the game's Scavenger World.
 * The defilement of the Golden City by Tevinter Magisters in the Dragon Age series, which gave rise to the First Blight and placed the future generations into constant danger of a Darkspawn invasion.
 * This is the case in Dark Souls. The First Flame, the source of all fire and light and disparity in the world is on the verge of dying. This is actually the second time this has happened. The first time happened about 1,000 years before the main events of the game, and caused the loss of the two most proactive Lords, Gwyn and Izalith, the complete downfall of the city of Izalith, unleashed demons onto the world and set in motion the events that caused the gods to abandon Anor Londo.

Western Animation

 * The Land of Ooo in Adventure Time is implied to be our world After the End, and there is mention of a "Great Mushroom War". This explains why there are electronics in an otherwise Medieval Stasis fantasy world, and the presence of Ruins of the Modern Age.
 * Generator Rex: A massive Freak Lab Accident infests the entire world with nanites, which cause extreme mutation on much of the population. Most of the major characters are implied to be somehow connected to the event.

Web Original

 * Open Blue has two: first was the fall of the Iormunean Imperium 1200 years ago, which is directly responsible for the existence of at least one major power in the setting, and indirectly responsible for another. The second was the more recent Disaster of Nations 3 months ago, which is responsible for the weakening of the world superpowers, and the return of the Pirate Lords to power.