A mega-dungeon is a conventional Dungeon Crawling scenario writ large, a place of danger and mystery on such a huge scope that it can fill the needs of a story entirely within itself. Indeed, some are so vast that they comprise the entire known world, at least for their inhabitants. As such, most of the examples you'll see are the primary focus of their works.
The concept of a dungeon has expanded with fantasy literature, and that applies to the mega version as well: it needn't be merely a complex of rooms beneath a castle. It can be a cave system, a complex of strange dimensions, even the interior of some massive creature. Because of its sheer size, a mega-dungeon can potentially be confused with a mere adventuring environment, especially if its origins are implicitly or explicitly natural or spontaneous. The difference is that, being a dungeon, each area within contains some particular obstacle, challenge, or threat; and that the structure as a whole has some singular purpose.
The mind-boggling scale of a mega-dungeon invites the Driving Question of who built it and why; usually Precursors are involved, and the place may serve as a massive prison for for some Ultimate Evil, or a vault to contain some immeasurable treasure. Either way, it will attract no shortage of adventurers ready to plumb its depths in search of answers. Even if its purpose is not known, it will gain renown far and wide; the greater the mystery, the greater the impulse to uncover it. But of course, the deeper one goes, the greater the danger becomes, both from the hazards inherent to the location and from other adventurers of great power who have also made it this far and are desperate to weed out the competition.
Another consequence of its size is that the mega-dungeon is more likely to have a permanent habitation of non-monsters. Some sort of settlement may be found at the base level of the structure or just outside it, providing wares to the steady stream of adventurers and buying whatever treasures they bring out. This is the classic Dungeon-Based Economy. Similarly, there are probably smaller settlements further in, functioning as rest stops or Dungeon Shops.
Like most related tropes, the mega-dungeon originates with the ur-Dungeon Crawler, Dungeons & Dragons. As dungeoneering made up the core of that game's experience, some developers eventually decided to cut out the middleman and create modules that contained nothing else, compacted into a singular location. This has in turn influenced fantasy works going forward, such that mega-dungeons are now commonly found in video game RPGs and stories based on them. In particular, a large (if not infinite), randomly-generated hazardous environment is one of the core traits of the Roguelike genre, which is generally built according to a consistent theme for simplicity.
- Though not a Dungeon Crawler in any conventional sense, Blame is set in a mega-dungeon of sorts: the superstructure is a functionally infinite world of concrete and random architectural features, built by maddened construction robots with no masters. Set long After the End, its inhabitants are all transhumans trying to eke out an existence in a chaotic landscape that neither acknowledges their presence or favors their survival.
- While the concept of "dungeon" - a magical ecosystem created by magicians and spellcasters - is a nebulous idea in the world of Delicious in Dungeon, most of the story is centered around the dungeon that was once the Golden Kingdom.
- The universe of Is It Wrong to Try to Pick Up Girls in a Dungeon? centers on the dungeon simply called the Dungeon. How deep it is is not known; the deepest level mortals ever explored is the sixtieth floor below ground. The Dungeon supports a Dungeon-Based Economy because the monsters within contains magic stones, which can be used as a renewable power source. This makes Orario, the city built directly on top of the Dungeon, an extremely large city.
- The titular Abyss from Made in Abyss is a multi-layered cave system that is regularly challenged by explorers for its relics and unique wildlife. Just the known locations are deeper than any discovered place on Earth and from certain point it becomes a guaranteed one-way trip.
- The Tower of Druaga: A sky-reaching tower full of monsters, periodically invaded by hopefuls seeking to reach the wish-granting artifact at the very top.
- In The Death Gate Cycle, the Labyrinth is a mega-scale dungeon where the Patryn were imprisoned by their rival demigod-like beings, the Sartan. It took them generations to reach the final floor and the exit to the rest of the world(s).
- Castle Gargantua is a "gonzo fantasy" mega-dungeon using procedural generation that boasts its own simple system. The dungeon is themed after the Seven Deadly Sins.
- Dungeons & Dragons has its share of mega-dungeons, usually intended to be the focus of most if not all of a campaign. These frequently have complex internal politics and a specific Story Arc.
- The Ur-Example is Maure Castle, created by Robert Kuntz in 1972 for a campaign that boasted Gary Gygax as a player. It was finally revised for publication in Dungeon Magazine #112.
- One of the earliest mega-dungeons was Dave Arneson's The Dungeons of Castle Blackmoor, originally comprising 10 levels. It was later published by Zeitgeist with ten additional levels added.
- Castle Greyhawk, the module WG7 version, is a mega-dungeon Played for Laughs. It comprises twelve comedy levels, including a send-up of The Wizard of Oz, a level that parodies other 1980s roleplaying games, and a temple of undead where the biggest threat is bureaucracy.
- The original Castle Greyhawk (which only had two joke levels, published separately as Dungeonland and The Land Beyond the Magic Mirror) was reconstructed by Joseph Bloch under the title The Castle of the Mad Archmage for OSR systems.
- Undermountain, also known as the Dungeon of the Mad Mage, is located directly under the city of Waterdeep and comprises twenty-three levels. It contains everything from a Harry Potter-inspired magical school to a portal to a Magitek space station.
- Rappan Athuk is a mega-dungeon that has moved through several publishers and is available for a wide variety of editions and game systems.
- The World's Largest Dungeon, published by Alderac Entertainment Group, comprises 1600 rooms on a single mind-bogglingly large level. While the idea is ambitious, the lack of interaction between neighboring areas and poor game balance made it controversial at best.
- Dwimmermount, currently available for several OSR systems, comprises 13 levels divided into sub-zones that interact in unexpected ways.
- The Anomalous Subsurface Environment is a "gonzo fantasy" mega-dungeon mixing sci-fi and fantasy tropes, though only three levels have been published so far.
- High-concept science fiction game Heroines of the Last Age takes place in a universe where the Lensman Arms Race is in full swing, and its player characters are staggeringly powerful. Accordingly, its "dungeons" are Big Dumb Objects the size of planets or even stars.
- The Oubliette roleplaying game pitches a mega-dungeon as its sole setting, serving as a prison for gods, spirits, and immortals. As the name implies, it is a place such beings are dumped to be forgotten.
- The Legend of Zelda:
- The Legend of Zelda: Phantom Hourglass: The game's progression consists of finding items in dungeons in order to progress further into the Temple of the Ocean King, obtaining what is necessary in order to go and clear more dungeons. There are other objectives and Plot Coupons along the way, but the Temple of the Ocean King is ultimately the most crucial location in the game, as it's there where the Big Bad (Bellum) is located.
- The Legend of Zelda: Spirit Tracks has the Tower of Spirits, a very tall dungeon (and the tallest one in the Zelda franchise with 30 floors, surpassing the 13 of its predecessor's equivalent) whose magic is connected to that of the rails the trains travel across (the rails also have the function of sealing the Demon King Malladus). As Link and Zelda venture through the temples of Hyrule to restore the rails, they also manage to rebuild the Tower and explore the higher floors, as doing so allows them to explore new parts of the land to access new temples and repeat the cycle. Unfortunately for them, by the time they fully restore the magical seal, Malladus is already freed by Chancellor Cole, so the purpose of the last round of the cycle (which consists of the last remaining temple and the Tower's highest floors) revolves around finding the weapon that can kill Malladus and then gain access to his new location (the Dark Realm).
- Legacy of the Wizard takes place almost entirely in a single dungeon 16 levels deep. With there being four very, very wide rooms to each level and lots of hidden passages, it's easy to lose your way down there.
- The Zelda Classic quest To The Top revolves around a boy climbing a ten-level dungeon, each with its own gimmick. The gimmick of Floor 9 can be treated as a game on its own, acting more like a traditional Zelda adventure with a set of dungeons, complete with a Bonus Dungeon accessed under special circumstances.
- Gauntlet clone Demon Stalkers is an Action RPG mega-dungeon comprising a full 100 levels, with lore telling the story of a previous unsuccessful attempt to kill the demon Calvrax imprisoned on its bottom level. It contained a Level Editor that could be used to make your own mega-dungeon.
- The Enchanted Cave has the titular cave, which has 100 floors in the original and 80 in the sequel (100 in the sequel's paid versions), to which people seemingly go and end up missing.
- Etrian Odyssey: Generally, games in this series are based around a single massive dungeon located within the fabled Yggdrasil, with a town at its base, from which adventurers go forth to map the environment and uncover its secrets. Typically, the labyrinth consists of 30 floors distributed in six strata (dungeon labyrinths), each with a specific theme and gimmick. In the third game, it has only 25 floors, with the first five strata having four each and the sixth having five (the game makes up for this by having a playable overworld map, which is set in an ocean). The fourth and sixth games only have their respective final strata (Forgotten Capital in the former, Yggdrasil Labyrinth in the latter) set within the tree proper, thus averting this trope (in both games. the main strata are located in the overworld, though they're still magically linked to the tree).
- Fate: A dungeon of infinite floors, with a quaint town at the surface. You can try to make a name for yourself by hunting legendary monsters somewhere within its depths.
- Legend of Grimrock: The first game is set in the interior of Mount Grimrock, which has been hollowed out into a thirteen-level dungeon. Criminals, including the player party, are thrown in at the top and can earn their freedom if they make it to the bottom.
- Lufia II: Rise of the Sinistrals: The Ancient Cave, 99 randomly-generated levels of increasing difficulty beneath the town of Gruberirk. Also, you have to give up your current gear when you go in, to retrieve it when you return; but you can find some very powerful items inside, which you get to keep. The Final Boss is a giant slime who congratulates you and offers a huge reward if you can defeat it within a handful of rounds. The game has a New Game+ mode that lets you choose any set of characters to venture into the dungeon until you get to the bottom.
- Persona 3 has you explore Tartarus at night, which is a 264-floor tower divided into six blocks with an optional 10-floor underground dungeon.
- Torchlight provides a reconstruction of this trope with its 35-floor story-driven dungeon, which is essentially one continuous passage from top to bottom. It is eventually revealed that the dungeon was created because the location was populated over the eons by multiple different civilizations for different purposes, but due to the machinations of the Big Bad residing at the bottom floor, all of these civilizations failed, fell into ruin, and were lost to history, buried by the passage of time, with a new civilization eventually establishing itself directly over the ruins of the previous civilization, before ultimately suffering the same fate. The eponymous town is the latest civilization to be built on the location, and known for being the largest cache of Ember; at the time of the player's arrival, it too was beginning to suffer the ills that befell the past civilizations previously built there.
- Fallout 3 has the DC Metro Area, a collection of above-ground sections inaccessible directly from the main over-world pertaining to real-life DC metro-area locales (the largest of which is the National Mall) and with a number of interiors for aboveground ruins in the areas, connected by a crisscross of subway tunnel ruins. Some (potential) friendlies within the mega-dungeon are a couple Brotherhood of Steel outposts and a colony of Ghouls that functions as an in-dungeon town.
- Angband measures its dungeon depth by feet underground, not numbered levels, with players traversing randomly generated levels of the fortress Angband to destroy Morgoth. Angband extends infinitely down, but Morgoth is generally found about a mile straight down.
- Similarly, Crypt of the NecroDancer. Since the player characters died there and are more or less haunting the place, they cannot ever leave until the NecroDancer is dead.
- Diablo: Technically, the whole game takes place in Tristram's Cathedral, which has 16 levels divided into four distinct tilesets. The place gradually appears to tunnel into Hell itself thanks to Diablo's influence, but the transition is such that it remains the Cathedral the whole time, surprisingly huge for a relatively small town.
- In Dungeons of Dredmor, your character comes along with some class-specific gear and never sees the outside.
- NetHack is set in the Mazes of Menace, comprising around 50 levels, which your intrepid adventurer (or tourist, or caveman) enters to recover the Amulet of Yendor from beyond the Valley of Gehennom and offer it to the gods.
- Pokémon Super Mystery Dungeon has Destiny Tower, which is the largest dungeon in the game and is unlocked after connecting with Zygarde. It has ninety-nine floors, with every five floors housing different pokemon of common types like Poison, Fairy, Steel, and Bug, making it a difficult dungeon to traverse and plan for.
- The Dungeons of Doom, home to the original Rogue, is infinitely deep, but the Amulet of Yendor is generally found around level 26-30. You can't go up levels until you've recovered it.
- The Family Castle in Rogue Legacy is where all the action of the game takes place, except for your Hall of Records where you can view the Family Tree.
- Scarab of Ra takes place in a procedurally generated pyramid, starting at the top, so each level is larger than the previous one. Due to interface limitations, there is presumably some cap to the level size, but you will run out of food and other necessities before reaching it.
- Deadly Rooms of Death: King Dugan's Dungeon is the eponymous setting of the first game in the series. It was originally built nine levels deep, but has expanded to fifteen and then twenty-five levels; Beethro's goal is both to clean the dungeon of monsters and to discover the cause of this expansion.
- Delve takes place entirely in the eponymous area, a sprawling, 99 (plus an unspecified amount of pocket dimensions)-levelled mega-dungeon. There are monsters who have set up entire countries, such as hostile lizardmen, an Apocalypse Cult dedicated to the Devourer of Worlds, and a Proud Warrior Race of (all-female) minotaurs who are friendly under the right circumstances.
- Tower of God: The Tower seems to comprise everything, to the point that for most people the sky is a myth. Most people live on the bottom levels, while climbing to the top is a highly dangerous task that only the chosen few or those of great power can even attempt.
- The Gearworld is an incomprehensible labyrinth of concrete and rusted gears, most of its structures having no apparent purpose, and the place as a whole creates the impression of being some sort of organic growth more than the result of some civilization's intentional efforts. It has entrances on opposite sides of the world, but it's unknown whether it's just that big or if its Alien Geometries allow it to be in several places at once.