What's that, you say? You're from another dimension and have no idea how you got here? Don't worry, that happens all the time! We blame the Gods.
(Popular ways to become a Dungeon includes Divine Recruitment, Summoning rituals Gone Horribly Wrong, Necromancy Gone Horribly Right, reincarnees getting misfiled by the Celestial Bureaucracy, or simply people dying while in contact with the right kind of magical rock. Oh and occasionally some Dungeon Cores are just created without any soul shenanigans at all, but those guys are kinda stupid and uninteresting.)
Where was I? Yes, from now on, you are the Genius Loci of a Dungeon. Inside your Domain - which you can expand, with effort - you can rearrange your corridors and place traps for unwary intruders! You can also summon monsters to guard yourself, evolve them into new forms, and promote some of them into Bosses.
As you grow in power, you will be able to unlock more Floors in your Dungeon, each of which can be given its own theme. A high level number may unlock additional options. As you dig down, the increasing Mana concentration will allow for placing ever-better traps and monsters. Remember to keep your vulnerable Dungeon Core in the very bottom of your Dungeon, guarded by your most powerful Boss Monster, to keep it safe!
The downside is that your editing power fizzles out in the vicinity of intruders, and also you will be metaphysically unable to wall your core off - there must be a traversable path from the surface to your core. (Of course, it can be made arbitrarily unsafe to travel...)
Now, typically, the three resources of a Dungeon is your Mana, which is absorbed passively from the world and used to perform work, your Mass, the elements you absorb from the world and used to build with, and your Dungeon Points, which grows as you absorb items and bodies, and can be used to research new creation options.
Dungeons often have particular over-arcing themes; this can affect the type of monsters, loot, and challenges available - and naturally, the type of adventurers they attract. Picking a starting monster type is usually the first step toward picking a theme, though sometimes Titles and Skills the Dungeon Core earns via consistent effort will be just as important. Note that picking Undead or Demonic starter monsters - or just starting to expand on the surface instead of digging straight down - is a good way to be marked as a threat and purged by the local inquisition!
You're probably going to be at odds with Adventurers, but don't go thinking you're some kind of Absolute Evil because of that. See, Dungeons are important structures in the world, essential for proper Mana circulation, and they are also convenient places for humans to practice adventuring. Well, the humans you don't manage to kill and recycle into Dungeon Points. It's a symbiotic relationship, kinda. Dungeons provide vital resources, and attract adventuring tourists, so expect a city to form around your Dungeon, and surrounding countries to try to claim or conquer that city!
There are ancient and powerful Dungeons in the world, with uncountable floors, able to take human shape to interact with visitors, and change the laws of physics themselves within their corridors. Some say that the sapient races were originally created as Dungeon monsters, escaping and multiplying. Others say that the world itself is but a floor in such an ancient Dungeon, the Gods its Mini-bosses. This is what you can aspire to - but that is only if you don't get crushed by a Ragtag Bunch of Misfits with rusty swords in your first week of existence!
Originally pioneered by Dungeon Keeper and similar games, A Dungeon Is You is by now a staple literature genre on Royal Road, and also occurs in various Quest Threads The concept shows up in Light Novels as well, but there the MC is usually a Dungeon Master controlling the Core, rather than the Dungeon Core itself, and as such these stories fall under the Dungeon Maintenance trope.
- Dungeon Keeper Ami, one of the earliest examples of the genre, places Ami from Sailor Moon in the world of Dungeon Keeper, where she is forced to take on the job of a Dungeon Keeper.
- Also the AU fic, Dungeon Keeper of Love and Justice
- Dungeon Hulk centers on a Dungeon Core reincarnee that suddenly appears on a chaos-infested shipwreck pile-up in the Warhammer 40K universe.
- I Woke Up As A Dungeon Now What has Taylor walking up as a dungeon after the events of Golden Morning.
- In Ancient Dreams, the MC, a summoned demoness, is bound into a crystal by a summoner with a grudge and left to slowly fade away. Hundreds of years later, she manages to make herself a Dungeon Core, having only dim memories of her previous life.
- Dungeon Core Trilogy focuses on a wizard who has been transformed into a Dungeon Core.
- The Divine Dungeon sees a victim of necromancy bound in a crystal develop Dungeon Core powers and become a Dungeon core.
- Bunkercore is a Sci-Fi example, with the MC expanding his base with nanobots following LitRPG rules.
- Dungeon Keeper: The player controls a "Keeper", a bodiless Sorcerous Overlord whose only physical presence is an immobile stone Dungeon Heart that sustains your life. Your minions claim territory in your name, which you can instantly furnish into various dungeon rooms that attract and support a force of evil minions. You can build rooms and traps, cast spells within your territory, and relocate your own forces, but otherwise rely on your minions to accomplish your goals. Some levels pit you against rival Keepers.
- War for the Overworld: A Spiritual Successor to the venerable Dungeon Keeper series, uses the same format as that game (as well as the originally planned name for the third game in the Dungeon Keeper series).
- Dungeon Maker: Hunting Ground and its sequel Dungeon Maker II: The Hidden War both subvert the trope, casting the player as an architect who builds and furnishes a dungeon specifically to hunt the monsters that are attracted to it, with the goal of eventually luring in the most powerful and dangerous monster in the setting.
- Blue Core  takes place in a setting where powerful amoral mages use their pet Dungeons as siege weapons and soldier-spawners , resulting in a Crapsack World. Occasionally nsfw.
- In Doomed Dungeon , the MC is made by a high-level Necromancer intending to come back and harvest him for materials in a few months. If he wants to escape that fate, he needs to power up A LOT before then...
- Dungeon Heart  sees an elderly Dwarf fashioning a Dungeon Core crystal, inhabiting it after death, with the intent of making sure his exquisitely crafted items will only go to the worthy from now on.
- In I Was Reincarnated As A Magic Academy! , a Magic Academy experiments with making an artificial Dungeon Core from core shards, in order to lower maintenance costs. The MC's soul somehow ends up inside...
- In Saga Of The Soul Dungeon , the MC's soul is summoned and fused with a Dungeon Core as part of a magical experiment, meaning that in order to found a Dungeon, he must first escape a Wizard's laboratory...
- Silverglade A Dungeons Tale  has an extreme introvert reincarnate as a Dungeon Core, decide that this is the life - she never has to leave her room again - and then proceed to screw up in interesting ways.
- The Misplaced Dungeon  has irresponsible Gods clone human souls and mindwipe them to function as Dungeon Cores. The MC manages to escape the mindwipe and establish a link to her clone-self on Earth, making her a Patient Zero for infectious LitRPG magic, with global consequences...
- There is no Epic Loot Here, Only Puns  is a fairly light-hearted example, with a quirky Dungeon Core that decides to be peaceful and friendly - except she first has to deal with the hungry spiders, nasty mushrooms and undead abominations seeing her Dungeon as free real estate.
- In The Dungeon Calls For A Sage , the one reincarnated as a Dungeon is... a former Dungeon Core. Disillusioned with existence, the core decides to make itself attractive to wise men and sages, so as to meet someone qualified to solve its philosophical dilemma and find a purpose to its life.
- In Travelers,  a group of friends are transported to a Fantasy setting by a Scrying spell gone wrong, and one of them is made into a Dungeon Core.