“Mystery Dungeons” are the colloquial name given to a natural phenomenon which occurs in areas with excessively high Mysteriosity. It is understood that within the boundaries of a Mystery Dungeon local Mysteriosity has reached a level sufficient to begin messing with both space and time, but exactly how this process works or why is almost entirely unknown.
Mystery Dungeons are best identified by the intense feeling of being in a significant location which occurs while near them. While it is not unheard of for Pokémon to wander into Mystery Dungeons with low Mysteriosity without noticing, Mystery Dungeons with higher Mysteriosity feel more intensely important and are harder to mistake. This effect increases the deeper you go into a Mystery Dungeon, with the most noticeable increase occurring the exact moment you step inside.
Inside a Mystery Dungeon, local geometry is effectively randomized. A Mystery Dungeon will always resemble the area’s true appearance prior to the Dungeon’s formation, but the exact details and layout will change on a regular schedule. The exact cycle varies depending on the Mystery Dungeon in question but is usually no shorter than 24 hours, although Mystery Dungeons seem to prefer to remain consistent while being observed. It’s recommended not to sleep inside of a Mystery Dungeon for this reason.
Mystery Dungeons naturally assert their own rules onto reality. The extent of the possible behaviors exhibited by a Mystery Dungeon is entirely unknown, although it is theorized that any Mystery Dungeon with increased Mysteriosity will have more noticeable changes, and the only way to know precisely how a particular Mystery Dungeon behaves is to enter it. This makes Guild organizations such as the Vanguard Branch extremely important to both safe expansion into unexplored land and safely containing the fallout of a new Mystery Dungeon appearing in an inhabited area.
Once a Pokémon enters certain Mystery Dungeons it can become difficult to exit again through normal means. Most often the entrance used will simply disappear, or in more open Mystery Dungeons space may distort itself to push any Pokémon trying to exit closer to the center. The only consistent ways to leave a Mystery Dungeon determined to hold you inside are to use an item capable of warping the holder outside, such as Escape Orbs or some Rescue Badges, or to reach the epicenter of the Mystery Dungeon and exit from there.
Dungeon Items
Certain items have a tendency to spontaneously manifest inside of Mystery Dungeons. Often these items relate to the Dungeon’s origins. For example, a Mystery Dungeon which manifests within a farm or orchard will typically contain the foods which grow there. Other items, such as Wonder Orbs or Poké, appear consistently across virtually all Mystery Dungeons. It is not known why this happens.
Due to the fact that a fortunately placed Mystery Dungeon is a vastly more effective way to gather vital resources such as food than farming or foraging in non-Dungeon areas, nearby Mystery Dungeons typically play a major role in the success of a newly settled town and in the lives of local Wild Pokemon.
Dungeon Pokemon
TODO
Temporary Note: Mystery Dungeon Pokemon
As for dungeons my working answer is “it depends.” Dungeons are known to be both confusingly laid out and a source of resources, even if wild pokemon haven’t yet decided to resolve the issue of property by trading the strange metal disks that appear in them and rarely understand the usefulness of held items, and so you’ll come across a lot of wild pokemon in dungeons who have come there to hide or scrounge for food and berries, especially if the dungeon is known to contain a large amount of food or berries. But dungeons are also known to create pokemon wholecloth from time to time, although the extent varies from dungeon to dungeon. Dungeon-created pokemon are arguably just as “real” as the coins you’re picking up, but they can’t leave the dungeon itself and disappear shortly after fainting.
https://discord.com/channels/1325678677727383653/1325678678264385660/1326962480089530368