Prequel in the Lost Age

In a series set After the End, especially if it's so long After The End that the Golden Age is nothing but myth and legend, one constant factor is that the Golden Age and its end will be tied in with the plot somehow, whether this means Lost Technology for the heroes to use, or a Big Bad who is a Living Relic from those times and is plotting to bring them back at any cost. However it's played, tantalizing details about that time will be revealed, and if they're interesting tidbits, they'll whet the readers' appetites for more. As a result, one of the most popular settings for a Prequel is during the lost golden age, before the world-shaking cataclysm that set up The Verse that we know today.

Most of the time, the prequel will prominently feature said worldshaking cataclysm as its climax; indeed, it's almost mandatory, so that viewers can see how the wondrous world of the Golden Age became the world of the original series. Normally, the Golden Age will be (at least superficially) a Utopia filled with either Science Fiction trappings or Magitek; Crystal Spires and Togas are often shown. It's very probable that the setting will be a Crap Saccharine World under the surface, in which case it's also a Soiled City On a Hill. Prequel in The Lost Age can overlap with Just Before the End, but this is actually somewhat uncommon; normally, everything seems to be going smoothly right until the lights go out.

Warning: Late Arrival Spoilers will be unmarked.

Fanfic

 * This is a common trope in Sailor Moon Fanfic, exploring the original Silver Millenium and its downfall before the series started.
 * Also common amongst Touhou fics, set before the establishment of the Hakurei Barrier. Since some characters are Time Abyss, this is fairly easy. How the characters worn Elegant Gothic Lolita outfit during Heian era is never pointed out.

Film

 * The Star Wars prequels are set before and during the Clone Wars and the downfall of the Old Republic, and feature the Jedi Order as it was before the Purge. The time difference is much smaller than usual, but the trope still applies.

Literature

 * The Heralds of Valdemar universe includes the Mage Wars trilogy, the first book of which is set during the titular conflict and leads up to the Cataclysm. The other two books deal with the immediate aftermath and set up the state of the world to come in the main chronology, 3,000 years later.
 * The Corean Chronicles have a prequel trilogy set in the time of the Duarchy, starring
 * The Star Wars Expanded Universe includes many novels and games set in the time of the Old Republic, usually thousands of years before the Battle of Yavin.
 * When The Tripods Came is a prequel to The Tripods.
 * The "Dunk and Egg" stories are an ongoing series of prequel novellas to A Song of Ice and Fire set about a hundred years or so before the start of that series, during the tail end of a prosperous period under the old regime. They are somewhat Lighter and Softer and generally end on at least a bittersweet note.
 * In the Dragaera series, the Paarfi Romances are a fictional in-universe series detailing what the in-universe author considers the "good old days", where nobles were brave and honorable and peasants knew their place. In contrast to the modern, First-Person Smartass style of the Vlad series in the "present", these are written in Antiquated Linguistics (although interestingly, they are written in-universe some time after Vlad).
 * Isaac Asimov wrote the Galactic Empire novels, set within the First Galactic Empire prior to the collapse depicted in the Foundation trilogy, as a bridge between his Robot series and that trilogy.
 * Katherine Kurtz wrote the Camber trilogy and the Heirs of Camber trilogy to explain the state of affairs in Gwynedd during The Chronicles of the Deryni trilogy. Specifically, the Camber and Heirs of Camber books explain how a land that had humans and Deryni co-existing openly (including Healers as a regular feature of then practice of medicine) became a land where Deryni often had to conceal their abilities or face death.
 * In the Co Dominium series, Falkenburg's Legion is a prequel to The Mote in Gods Eye and King David's Spaceship. Set during the CoDominium era, it focuses on a mercenary band's attempt to stablize off planet colonies before World War III devastates Earth.
 * The Horus Heresy novels are essentially the story of how Warhammer 40 K became such a Crapsack World.
 * The Great Migration Duology books (Crystal Soldier and Crystal Dragon) in the Liaden Universe.
 * The Forerunner Saga is a series of Halo novels set during the heyday of the Forerunners 100,000 years ago, when they were spread out throughout the Milky Way. All that's left of them now are artifacts on many planets, as well as a set of giant rings meant to wipe out all life in the galaxy.
 * The Magic the Gathering novel series has The Thran, a novel about the time before the five colours of magic were discovered. It contains the Start of Darkness moments for some of the series villains as well as explaining where several of the Plot Coupons used in the other books originated.
 * Some of the Dune prequel novels from Brian Herbert and Kevin J Anderson cover the distant past where sentient machines ruled the universe.
 * Darkwing is a prequel to the Silverwing trilogy that takes place in the prehistoric era.
 * The Bio Shock novel Rapture mixes this with a slide into Just Before the End. It basically links all the backstory we know about the first two games together into a single novel.
 * Most of Wizard and Glass is set Just Before the End of the time of the Gunslingers and tells of a time when Gunslingers still kept peace in the world, time and space were still somewhat dependable, and there seemed to be hope that even though the world had already somewhat "moved on," things maybe could still get better.

Live Action TV

 * Caprica is this to Battlestar Galactica Reimagined.

Tabletop Games

 * Exalted's Dreams of the First Age supplement is set in the titular First Age, a high-Magitek civilization ruled by the immensely powerful, and increasingly power-mad and deranged, elders of the Solar Deliberative. The supplement kicks off at the very moment that the Unconquered Sun turns his face from his Chosen.
 * Earthdawn is a prequel to Shadowrun, set in the Fourth World. Shadowrun itself is set in the Sixth World. (As a point of comparison, we're currently living in the Fifth World.)
 * Not much longer, the clock is running.
 * Rifts Chaos Earth is set during the Coming of the Rifts and the end of human dominion over the Earth. Ironically, due to the rarity of magic, the nonexistence of Techno-Wizardry and the fact that Chaos Earth hasn't been fighting a constant war for survival for centuries, the tech level is in many areas lower than that of the mainline Rifts Earth setting.
 * The Arcane Age line of Forgotten Realms products explored the days when Netheril and Myth Drannor were at the height of their power.

Video Games

 * Inverted in Phantasy Star II; it's a sequel to the first game, set a thousand years afterward in a world where technology has created a Utopia and its breakdown is driving the Algo system off a cliff. On the other hand, it also has a sequel of its own set after the destruction of Mother Brain.
 * Sagi's headache induced "Elsewhere" scenes in Baten Kaitos Origins takes place before the War of Gods. It only becomes clear at the 3rd occurrence, where the group notices the land goes on endlessly, in contrast to the Floating Continents the series uses.
 * Final Fantasy XII and Final Fantasy Tactics a 2 stand as prequels to Final Fantasy Tactics. Probably.
 * Wild Arms 4 seems to be set during a period of time constantly alluded to in other games in the series of a massive world war where cyborgs and nanomachine monsters freely roamed the lands. Many of the world's bigger cities have been destroyed in the chaos and are slowly turning into the types of locations you see in games such as Wild Arms 3.
 * The Elder Scrolls Online will be set in the Second Era in the Elder Scrolls franchise history, a good millenium or so before the The Elder Scrolls V Skyrim, and even before the empire of Tiber Septim was established.