Dungeons

Dungeons are created when a newborn Astral Core begins to take roots and expands its Domain, taking over its surroundings. They are an important trump card of every country (just as long as they doesn't go on a murderous streak, as it sometimes happens after they begin to be influenced by the powers of Pandaemonium or just grow up to have a really bad personality).
Dungeons as a locations are basically impossible to describe without describing exact mechanisms of how Astral Cores grow and how they control and exploit their Domain. After all there is no real diference between Domain, Astral Cores and the Dungeon - they are the same things, just differently named and slightly different in their nature. Astral Core creates Dungeon by spreading its Domain, their personality growing more and more complicated until they are finally able to manifest as a Dungeon Master/Lord, a semi-independent entity into which their personality 'moves over'. Slowly beggining to think of itself as a separate (although connected) being than Dungeon as a whole, which sort of means they are reaching adulthood and soon will be able to reproduce (detail can be found here).

Assimilating

Oh great. Now, answer me. Who was brilliant enough to let a group of fire magicians explore our dungeon? You know, the one which our alchemists were using as a cheap source of herbal ingredients?

A mayor of a town with a dungeon after upper levels were turned into
fire-based locations, with fire elementals burning out plants.

Dungeon's are capable of assimilating a wide variety of things to fuel their own growth. They can consume all matter that is left alone for long within its borders, including organic one as long as it is not alive. Assimilated matter opens new possibities for Astral Core to evolve, allowing it to manifest new things (objects, beasts, puppet-mortals and so on). Dungeon Hearts (another, local name for Astral Core) in most cases inherit some of the basic feelings of aesthetics from their parents (thus they now what corridor, room and so on are).
Dungeon that assimilated enough fishes and water can create an undeground lake inhabited by fishes - while it does know more or less what 'lake' is from the start, it still needs to learn to materialize water and things it want to populate the lake with, via analysis of assimilated flesh. Not to mention that the old Law of Magicks that the more something is complicated the more magic power it requires to be done still stands.
It can also assimilate things immaterial, like knowledge, abilities and spells. Which work differently than typical, automatic absorption. - it has to be done willingly. Dungeon Master needs to WANT to absorpt them from the creature that was slain in its Domain. The problem is also that in long-term skills (like knowledge of alchemy or fire magic) the progression is flat. You can't just steal all the knowledge from a masterful fire magician, killing and assimilating him will give you several spells of his, but they will be unusable because it won't be accompanied by the neccesary knowledge about fire magic. Which will require 'eating' more of its users over time, to be able to make sense of the spell he consumed.

Construction

Dungeon growth confirmed. It seems to have finally created 20th Level. According to scouts, the new expansion is mostly similar to the earlier levels, but we do found several interesting herbs that weren't there earlier.

Raport from an adventurer group.

Dungeons are separated into Levels, Rooms and Halls. Levels are, well, entire levels, in most cases created more or less to become a habitat for a certain types of creatures that require specific environment. Rooms are 'cells' into which a Level is divided, made with certain goals or work in mind. Some can include a field fortifications to stop enemies from proceeding deeper or lairs (later on more about them), other can be just simple rooms but with certain ruling 'theme'. Halls can be considered a sublevels, a large rooms with heavily compressed area that can house a quite big (and sometimes completely open) area, that are absolutely required if Dungeon wishes to house a proper foodchains, not to mention making places where adventurers can properly rest (some of them literally includes a small villages).
Cores grow overtime or, as it was said, through consuming life force of creatures it 'devoured'. Mortals have more of it, animals less. In terms of beasts it depends on the strenght of the creature. Most of them - those that were assimilated thoroughly enough - can be then 'spawned' and used to populate the Levels and Rooms. There is also a possibility of using them to produce more magic power by extracting it from the 'veins' beneath the ground. They can be mined indefinitely, but with the ammount of power gained through this heavily dependant on the proximity to Core and its strenght, as it draws power unconsciously, making these veins more concentrated.
It is hard to exactly describe the processes that happen in the Rooms dedicated to aether-extracting. It is believed that most of unbound ground aether is transformed into liquid form and consumed by the Dungeon to power up its growth. The rest is taken by adventurers that reached the extraction sites. There is also some unbound but not processed into liquid aether, that is radiated to the outside, making nearby areas slightly more magical (while also slightly rising the general level of magic in the world).
The bigger the Domain, the bigger the possibility of storing power though there are also certain Room improvements that can increase the storage ability. Regenerating it depends on life force consumption, aether extraction and belief about the power of the Astral Core.
Dungeons are categorized by Grading system. There is one Black Crystal level Dungeon beneath the Imperium of Karadia's capital, that can easily go to 200 Levels - according to legends, its Astral Core is the one that is mentioned to be created directly by the gods, making it a Demigod-like creature. Black Gold are only a few (and each of them can reach 100 Levels) that are supposedly descended from the first Dungeon. Next levels are, similarly, another generations, until the Copper Grade that can barely made 10 Levels, are barely capable of making some interesting things... and are also most widespread.

Materialization

What? I haven't stole your sword, you dimwit. I just took it from the same Dungeon.

Adveturer accused of stealing from his compatriot.

Ammount of power that the Dungeons can gather and store is absurd from the mortal point of view. Through that overwhelming power they are capable of doing several things that humans are completely unable to do. Practically reaching the realm of divines when it comes to manipulate and create things.
Dungeon Hearts can materialize both items and living creatures. With enough power it is completely possible to materialize for example a spear made from materials that the Core managed to assimilate and analyze properly. Of course, first such creation will be of most inferior grade, but after being able to take a look of decent examples of them, the quality can grow impressingly fast. Of course, creating objects from more magical materials can be much more costly and requires assimilating and analyzing much more elusive materials first.
What's more there is no real problem with creating for example a lizard, wolf, dragon or human. All that is needed is enough power and understanding of their anatomy. Of course, creating a body doesn't mean creating the soul. Because of that, such creatures are little more than puppets - to some point intelligent but very limited on that part - of the Heart. They still have to be cared for, keeping them satisfied, but after leaving the Doman thy will soon dissolve. Forever.
Of course, with sufficient level of improvements they, too, can reach an almost normal level of... "complication" in their behaviour. The word 'almost' being a keyword. They might fool somebody for a short while and even develop some habits and preferences, but they are ultimately still merely a very complicated puppets from meat and bones.
Of course, creating such creatures costs. A lot, especially if one wants to make them really complicated. Not to mention that they have to be supplied with mana, otherwise they will cease to exist. Their power is also heavily dependant on the distance from the Core, which makes putting some sort of apocalyptic creatures near the entrance rather... impractical. With overwhelming majority of Masters placing the Core in the deepest, least unacessible place possible, the deeper the more deadly.
There are also Lairs. Designating a Room as a Lair of certain type of creatures in most cases means setting a perfect habitat for them, allowing them to rest, thus lowering the power required for them to exist (also by making the natural density of aether in the room greater and 'attuning' the inhabitants to it). While the creatures don't have to eat, they can do it, gaining some power from it. Sometimes they even can procreate, though it is much more complicated, and might lead to the creation of de fcto food chain and functional ecosystem. Which in most cases requires using Halls as centerpoints ofsuch ecosystems.
Most of such creatures can also be heavily modified. This leads to every Dungeon possesing a nearly completely unique ecosystem, with plants, animals, beast and even daemons of their own chosing. Some means of standarization are forced by the fact that mass production of certain schematics with some small changes at best (unique facial features and so on) is simply much more efficient and cheap than assembling all of them from scratch. Even a unique beings (like a guardian of a important key) are in most cases standarized simply because there doesn't exist 'respawn' for them and they will need to be produced over and over again.
Once materialized, things and creatures theoretically can leave the Domain, although there are limits. Things and parts of dead bodies/herbs aren't in any way limited (otherwise the potential economy benefits would be made nil). Creatures that are alive require to be filled with magic of the Dungeon, that works as time limit - after it runs out, the creature dissolves. What's more, the strenght of materialization is heavily dependant on the ammount of aether concentration, which is proportional to the distance from the Core. To find really high valuable stuff, you need to get deep.

Unique Servants

It was a tough work, but we've managed to slain Shadowblade, the Unique guarding entrance to level 20. Really tough work, but the reward was decent. The problem is, I doubt we'll manage to take him down again, we used pretty much every trump card we had so unless we think of something new... its gonna hurt.

Leader of group of adventurers after Dungeon raid.

Uniques are creatures that were given a certain variant of soul by the Dungeon Master. They are special, for they can not only grow in strenght independently, but also are bound to the Dungeon which makes them possible to be resurrected after death. They are practically bosses of the dungeon, guarding important locations or just being a companions of the Master (after all he is enough of a mortal being to feel loneliness). Which in most cases doubles as bodyguards.
There also exists the so called Named Ones, creatures made unique by being named by the Master. They aren't given souls, and have no real intellect (by default, they can be still made very complicated and surprisingly cunning and intelligent to the point of being allowed to speak), but can be recreated easily. They are also simply stronger than average creatures.
However, the Uniques can be made much stronger by giving them more magic power (soul strenghtens) and skills assimilated by the Dungeon. On the other hand, their bound soul means they are TRULY unique beings. If they die outside of the Domain, they die for real - their soul goes to the afterlife. While they can be then restored, new instances will be completely separate beings, much weaker and without their memories and personality. Named can leave and die without any serious losses for the Dungeon (although they will still dissolve after running out of power).
On the other side, having soul means having free will, which makes Uniques capable of rebelling, altough its rare. Named Ones do not have such possibility.

Exploitation

Established contact with Dungeon Master. Manifested as a dark elf female and approached us after we expressed interest in meeting with the Master while inside the dungeon. She is ready to sign a treaty allowing us to exploit the Dungeon in exchange for certain goods and protection against those wishing to claim the Core.

Adventurer after scouting mission to newly discovered Dungeon.

Majority of Dungeons live in symbiotic relationship with local civilizations. It happens so often that Imperium literally has created a 'blueprint' of typical deal between local government and newly discovered Dungeon, that covers majority of important matters allowing to establish succesful trading relationship.
The general outline of typical agreement includes acceptance for deaths of adventurers within the Dungeon. Imperium has nearly infinite ammounts of adventurers to sent to die in a pit, so a losses are generally acceptable, as long as the Dungeon isn't made to be TOO lethal. There are actually specialized groups of adventurers trained to participate in 'balancing' the whole place. Dungeons could, for example, be practically infinite sources of gold - while simultaneously breaking economy through inflation. Even overly big ammount of herbal ingredients would cause a sudden shift in local economy, which theoretically could lead to many people going broke. And then? What if they rebel?
Imperium - and its gods - like the peace and progress but one under control. Thus the Dungeons have to be properly introduced to the world. Limits of things that could be taken out of it, ability for Imperium to sometimes ban certain things from existing and so on. The Dungeons are to be dangerous, just not openly murderous, with people dying for their lack for attention (and mistakes), but not butchered relentlessly. Not to mention that its inhabitants leaving the Dungeon for little bit of arson, murder and jaywalking aren't exactly welcomed.
Besides, unless Master is an omnicidal maniac dedicated to powers of Pandaemonium, too high mortality isn't really good for him too. While he can ate a lot in short time, he will ten starve because noone sane will enter Dungeon that kills 95% of intruders (while 4-5% are acceptable, especially when 4-5% will also get filthy rich). The lowest levels are of course an exception, sometimes called Death Zones or Killzones - they are a last few Levels, made to be exactly as murderous as possible. They are there to ensure safety of the Astral Core - they are insanely valuable, so from time to time a groups of highly motivated (by money) people attempt such heists. Killzones are marked with warning signs - those that go there do so at their own peril.
Due to that symbiosis and general growth of number of adventurers in history, Dungeons are one of the pillars of the crawling imperial industrial revolution. Magical anomalies gave unique types of magic and daemons to capture and contract new mages. Dungeons are better at supplying the technomaturgic industry with alchemical ingredients and aether. In minerals both types tie.

Magic

Alright, the Dungeon Master is just a real fucking pervert. He made a long corridor and put a Law on it that forbids wearing clothes. You have to run through it, with your whole equpment packed into your bag... and if you aren't fast enough, you get teleported right into the fucking entrance! Filled with people! I swear, this is a violation of a treaty!

Female adventurer after failed dungeon raid.

Dungeons also possess a certain type of magicks called the Laws. It is considered a downgraded version of Absolute Magic, that allows them to set certain rules of their choice to the dungeon, forcing people to obey them. Through pretty supernatural way. They can work both as a passive setting, for example forcing people to not use weapons to avoid a logic puzzle, or for a more active one that actually replaces the need for machinery (for example by connecting button to door it opens without having mechanisms that could be somehow damaged).
Dungeons can also bind daemons to 'use' within their Domain. Wild ones by persuading them to come or by creating necessary habitat (for example nymphs will be born automatically after making a cave lake). Servile ones by making deals with their gods. All of them can be easily anchored to the Domain by magicks, making their resummoning a child's play. But one that of course costs mana.
They also passively use spatial magic to make themselves 'denser' - by compressing its territory it can fit more things inside and create a 40-50 Level deep Dungeon without going few kilometers beneath the ground and hitting Labirynth...