Classic fantasy gaming has a strong legacy of danger and death filled dungeons but sometimes the appearance of the third or forth dungeon can stretch what is believable and damage the verisimilitude at the table. A key concept in maintaining dungeons sense of belonging and rightness in the world lies having origins that are organic. Variations of those origins helps larger amounts of dungeons populate the world with less damageto the overall setting.
1. Protective Barrier – The purpose of a dungeon may be to defend the goal at its heart. Whether tower of an archmage or portal to a mythic realm. The nature of the dungeon becomes more deliberate and controlled. The natures of the dangers are meant to drive off or defeat would be invaders or seekers of the forbidden.
2. Prison – A dungeon of this nature is intended to contain a goal. This being the case the nature of the encounters are as keyed to the intended captive or captives. Dangers are informed by the the jailer the prison needs. An efreeti prince kept by a family of red dragons to grant them its wishes is a solid example, their shared immunity to flame both making sense and lending thematic consistency.
3. Training Facility – A “danger room” designed to test adventures, mages or any other number of disciples is a great reason behind a dungeon’s build. If created in recent memory or in a manner as to customizable, it is possible that these dungeons are able to be reactive specific characters and challenge them directly. GMs are cautioned in overuse of this customization or they may become overly adversarial in the eyes of their players. The end goal isn’t to reach the center but rather become stronger. These dungeons can be non-linear in the extreme.
4. Test of Worthiness – Similar to the Training Facility, but in this origin the dungeon is a test to evaluate those who would seek its goal. Challenges are meant to solvable if difficult, but may not turn a specific eye to a party of adventures, rather the challenges are often matters of known lore. Tests of immortality or trials of religious worthiness are common examples of this type of dungeon.
5. Breeding Ground – In this instance the dungeon is specific place for the breeding or creation of a certain type of creature. Often the dungeon is the only place the creatures can be bred. These creatures are often best when they have different stages of development. Swarms of larvae, dangerous nymph-states, molting juveniles and full-fledged adults can be used to create broad ranges and types of encounters as examples.
6. Life-Sized Diagram – A dungeon can represent the physical aspect of a magical rite or piece of artifice. The dungeon’s challenges can be a coincidental or part of the needed mystical reality for the dungeons purpose or design. Fire elementals that energize arcs of a grand summoning circle or bone golems that act as foci for an massive necromancy.
7. UN-Geon – These dungeons are more circumstantial. Complex terrain features, natural patterns or game trails through a forest. Natural occurences forming dungeon-like environments and separate areas for structured encounters. These dungeons tend to best be thematic to a specific location or event. Examples include a forest invaded by demons or icy peaks that hold a sleeping elder evil.
8. The Repurposed Dungeon – From abandoned sewers to crashed spaceships to magic schools gone awry, these situational dungeons can be among the most believable and compelling. The nature of the dungeon’s challenges come from effective breakdowns or wild magics. Evaluating the potential of these dungeon’s original functions and natures of the place or thing the dungeon comes from.
9. Planar Tangency – Some dungeons are momentary. The results of planar alignments when a regular town becomes infused with nightmare demiplanar power, turning citizens into monsters that cannot be slain lest the dreamers die when the conjunction ends. Abyssal corruption twists the old battlefields into new versions of demon-infused hellscapes. Attributes of the planar overlay should be made manifest blendings of characteristic of each source plane.
10. Mad Redoubt – A classic but one of the worst offenders for shattering verisimilitude, the madmans’s magical complex that he designed and built for… reasons. The simple fact of the matter is that the crazy doesn’t make sense and isn’t required to can erase a lot of the needs of verisimilitude but if you intend to rely on this type of dungeon a lot, you need a common origin of insanity or perhaps an iconic madman who designed more than one deathtrap.
A dungeons origins impact a lot of the overall momentum of your campaign make sure you spend the time to select the origins that support your story the strongest in the long run. Or maybe just blame the plane of Chaos use a bunch of random tables.
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"Find the Hero Within" - Lost Spheres Publishing 
Depending on your players and style of game, there is also the dungeon with no apparent “purpose” per se. Modern dungeons have to be logical for some reason – its because gaming has grown up, and this has pros and cons. The point of the purposeless dungeon in the meta is that the PCs have stumbled upon a hidden “other” – some dark secret of the world that lurks beneath (possibly literally) the facade of the world they think they know. By “dark” I mean like “dark matter” not necessarily “evil.” Like stepping into a faerie ring, this is a place mortals were not meant to be and the laws of the universe do not necessarily behave the way the PCs think they do. It doesn’t have to make sense as long as mystery and “other-ness” are preserved and exploration takes precedence over random hack n’ slash, and there’s plenty of unusual weird moments that take your breath away just imagining them and times where you scratch your head and get a headache trying to figure out “why??” This is the way it used to be back in the day. For gamers used to the way things work nowadays, a dungeon like this can prove to be a refreshing break if it only occurs once or twice in a campaign without breaking the fourth wall. Perhaps combine with Planar Tangency. Immediately it springs to mind that a plot can be woven around one that gets found over and over with entrances in different places when they’re needed for some esoteric reason, as in the Chronicles of Narnia. For an obvious example, see the old 1st edition module EX1: Dungeonland by Gary Gygax.
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