In the same way a realm of spirits can be thought of as a physical thing, a mind can be a physical thing too. Not on its own, of course, but one can enter it. One can use the gates of astral travel, or powerful spells, or shrinking powders, or a psychic bridge more powerful then most. Then, you step inside the head of a sleeping person and enter their dream as a passenger.
Of course, within a person's mind, things are unsteady. You are bombarded with memories, trains of thought, imaginations. People and objects become idealized- it is best if you only enter someone's mind if they do not know you, else you will be subtly changed by the experience. If the target of this intrusion thinks of you as an annoying braggart; you will become arrogant and useless as you sit back and take the credit for everything. Women grow extra cup sizes in the minds of men, and men become whimpering, doddering fools talking about their “emotions” in the minds of women. Such things are best avoided, and are simply troublesome for everyone involved. Every few minutes, the dream will shift again and again, following a train of thought to new destinations only partially connected.
The Labyrinth seems to almost have an inbuilt defense against intruders like yourself; certain beings manifest from the thoughts and dreams of the person. The religious have statues of their most beloved Gods here within their mind- watching over even this place. Nightmares and negative emotions are actual beasts here. Traps in the labyrinth slide down into trauma from the past, hidden just beneath the surface. Their imaginary friend from childhood is still with them; it is the Minotaur that wanders the halls. The person whose labyrinth you explore is very much master here- if they become aware of your presence within their mind and are conscious; even a novice can make things even harder and could forcibly eject you. Masters of psychic warfare and psychology may well be able to trap you within their mind-maze at their own desire; letting them torment you further or even killing the intruders with the sheer force of their will and imagination. Of course- to the intruders, the ire of the owner of the mind they are invading will appear as black storms, or sudden hoards of jeering townsfolk in an angry mob; you are the unwelcome monster here.
The Mental Labyrinth
Each person has a unique maze within their mind. It can only be entered with specific spells and a magical journey. The maze has walls and floors that mimic the person's own life and personality- the labyrinth twists and turns along with their memories and thoughts. The traits of the individual make up the Labyrinth's details and the details of the mind.
The walls of the labyrinth are the place where the person feels at most most comfortable. This may well be a childhood home, the clean walls of a sanitarium, or it could be the rough hewn walls of stone, present in the masterful minds of dungeon crawlers and treasure collectors. It is where someone feels most “in their element”. The light of the Labyrinth is also based on the person's own nature and secrecy. Honest men have honest hearts; and in their Labyrinths the air is practically glowing. You won't even need to burn a torch, it may as well lit a summer's day. But most normal people with a few secrets, and especially those of criminal background or the guilty, will have darker and darker dungeons. The Labyrinths of spy masters, secret keepers, and dark people will have thick fog and heavy shadow that makes it even harder to navigate.
Depending on the creature's Intelligence score, the Labyrinth's details and interior spaces are changed. Creatures with a very high intelligence will have a mindscape which is well established in detail and landmarks. The layout of the Labyrinth will be relatively stable, you can map it out. You can look at the walls and see cracks. The monsters in their mind, memories or imaginings, have plausible biology and anatomy. If you are invading the mind of a creature with lower Intelligence however, the labyrinth will be vague. Instead of mapping on a grid- map the maze as a “point crawl”, with only the major things sticking out. Inbetween these points of interest the hallways lack any detail and are simply suggestions of forms. The creatures too are affected; blobby approximations. You can cut off a limb and the resultant stump will be a cartoon with a bone and generic pink flesh around it. Mouths end at the throat; no digestive system is needed for such cardboard stand-ins.
Depending on the creature's Wisdom score, the maze will denizens with their own goals, personalities, and factions. Those with high Wisdom will know the connections between people, and there will be a deeper level of interaction with the creatures of the maze. This does not necessarily equate to benevolence or neutrality- but more to a consistent and real “personality” to the place. Higher Wisdom scores also indicate a deeper level of religious and spiritual understanding; puzzles involving symbolism and the deeper cultural subtext will be created within the mind. Those with Higher Wisdom are also said to have a “rich inner life” and will have a more beautiful labyrinth, though this combines with Charisma as for artistic talents. Those with lower Wisdom will instead have a demographically barren labyrinth; few if any personalities will spawn within their mind, their motivations banal and pointless. Low Wisdom people may have Labyrinths that lack any kind of personal touch; any given part of it won't seem to be a part of “them”, as they lack that will. Those with Low Wisdom will even suffer from their own monsters and censor-beasts lacking somewhat in conviction; after all, only a very religious, empathetic man will truly know the full depth of evil a Balrog lives and breathes.
Depending on the creature's Charisma score, the maze with become more or less powerful and difficult to escape. This seems backwards; those with high Wisdom but low Charisma will have artistic, rich personal dungeons with talkative manifestations. This is because Wisdom and Charisma are used for different purposes; Charisma is however about force of will and personality as an OUTWARD force- and those factors bring great power to the mind-maze, where as Wisdom is more an INWARD force, the power of ones own convictions. Those with Higher Charisma are better apt to change the people who enter their mind, subconsciously or not, and will have higher walls, stronger creatures, and more vivid emotions. Those with powerful Charisma will have rolling waves of emotions or memories that intruders will feel grip them as though possessed by them. Those with weak Charisma, on the other hand, will have less passionate and less powerful labyrinths as a whole. The traps lack the sharp spikes of spite, the jokes and references to their personal life won't have the edge of a well timed joke. Escape a Labyrinth of a person with low Charisma is relatively easy, simply slip out and be forgotten, where as one with High Charisma will drag intruders in even deeper and harder to escape their innate magnetism.
The Alignment of the individual is also important to their mental labyrinth. Those with a more Lawful alignment will have tight right angles, well placed walls, and symmetrical layouts, where as those with a more Chaotic element will have organic walls that curve, chambers of varying sizes, randomly sized doors and so forth. The Good to Evil axis is more abstract, since everyone thinks they're at least a little bit good on the inside or justifies their behaviors- instead Good and Evil will manifest more as clouds of energy; positive if more Good, and negative if more Evil. Because this in the imagination, the fogs of positive energy will heal wounds and bolster holy and white magic, where as the mists of negative energy will degrade equipment, deal damage over time, drain levels with extended exposure, and empower undead. Of course, like-minded individuals exploring another person's mental maze will be more at home and rewarded for sharing the same alignment as the owner; but Evil creatures tend to be backstabbing and not work well with each other, so in this case it makes sense that the evil mists still hurt other evil creatures; but they COULD act as a force that is used to empower dark spells regardless, or act as healing mist for intelligent undead and evil-high priests and the like.
Finally the being's age and Experience level will indicate the size, depth, and complexity of their labyrinth. Children, simple races like goblins and kobolds, or a grown simpleton will have only a few turns, their whole labyrinth being no more then a few rooms of their most important features. Adults, adventurers, and monsters with a few HD will be more complex, at least those with minds and histories. The biggest, meanest purple worm isn't going to have a maze in their brain at all, because they don't really have one. The immortals, elves and ancient wizards, have massive, sprawling labyrinths. These beings may even have different “wings” of the labyrinth, each one representing a different period of their lives or where they reinvented themselves. The entire place is filled with lovers who died of old age, forgotten events from history, treasures that have long since been melted down back into gold and sold off in the real world from ages ago. This is probably the best way to talk to a dead man from a history book; but it will not be them as truly as they were, but how the individual whose mind you are sneaking around in saw that historical figure when they knew them.
Every exploration turn, you are faced with a Manifestation Roll, which is equivalent to a wandering monster roll. Instead of the roll being based on how encumbered, slow, or loud your party is moving; it is based on how aware the mind is of your intrusion. If you are wearing colors that go against their own personal tastes, are members of races they do not like or are unused to, if you are acting against the flow of their mind and so on you are making them more and more aware of your presence. This makes it more likely that a roiling manifestation of their mind reaches you.
Instead of using a normal 1 in 6 roll to determine wandering encounters, consider using a “roll a 18 or higher” on a d20. This equates to a 15%, close to a 1 in 6 at 16.66%
This way, you can add more modifiers for smaller breaches in conduct; add a +1 for each infraction.
It should also be noted here that the Labyrinth is at least partially in the “subconscious”, meaning it isn't quite as variable or easily as to control as the raw mindscape and imagination of the person. If the person becomes aware of you, they can't just immediately wipe you out by imagining you disappearing into nonexistence, as long as you are in the Labyrinth. Of course, if the person is aware of you inside their mind, they can focus to give you a manifestation roll every single exploration turn, with greater levels of control to those who have more Charisma and willpower. Of course, even this isn't fullproof- people sometimes find surprises within their own mind.
Manifestation Table- Roll 1d6
 Memory. There are memories in the form of portraits (to remember faces), maps (to remember places), and objects (to remember events) scattered all throughout the labyrinth. But sometimes specific memories manifest directly as snippets; glowing ghostly phantasms act out the memory. Disrupting the memory will cause awareness to grow from the Labyrinth's owner, but can also permanently alter their perception of an event. Also if anyone who has gone into the other person's mind to invade their Labyrinth was in this event in the memory; then the memory will not feature a clone of them but they must act out their role in that memory as the owner remembers it. This can also lead to clues about the invasion of their mind; you remember someone meeting you at the fancy dress gala, but in your memory they're wearing bloody armor with swords and rags. To the educated and powerful, jarring events like this in memory that nobody else seems to be remember is a very telling sign of someone sneaking around inside your brain.
 Imaginations. These are beings wholly created from imagination, and may be either supportive of the creator of the labyrinth (sharing alignment, protecting it, etc.) or may be against the creator, as though from a tortured psyche or moody artist. Imaginations are similar to illusions; you can make a saving throw modified by wisdom to disbelieve them, but in the mindscape everything is partially illusionary, and as such the illusions can still interact with the world but simply cannot hurt you unless you allow them to. This still requires a successful save, even though you know they are “fake”.
Imaginations can exist as any kind of creature, person with any equipment, or even made up and fictional creatures which cannot or do not exist in the “real” world of the fantasy space.
 Moods. Moods appear as colorful ghosts or spectral versions of the labyrinth's master themselves- they always act according to the mood. Moods are stated as spellcasters, and can shift the labyrinth around them based around their emotional states. Moods get reaction checks when encountered, which indicates how reasonable or clouded they actually are based on their mood or emotion. Very low rolls will make an anger elemental attack you out of habit, where as a high roll would indicate a bit more nuance and being able to actually have a conversation.
 Static. Strange half formed ideas, boiling emotions, or half-forgotten dreams struggling not to die among the eddies of the master's mind. They appear like flaming orbs or cubes of shapes, with suggestions of shapes and voices. Less of a creature and more of a hazard; flies down corridors in a haze of change. Make a save to duck out of the way or take 2d6 damage.
 Custodians. These are like spirit guides, memories of teachers and mentors, positive spirits or ideas, or other usually friendly beings inhabiting a person's mind. While they are usually of inner origin, some of them may be actual angels putting good ideas in their heads, or ancestral spirits granting old knowledge or magical power by realigning the stones of the mental labyrinth.
Roll a reaction check when interacting with a custodians. Only if you are obvious or a bad result is listed will they realize you are foreign to the mind maze, and will otherwise be helpful, granting advice or directions.
 Censors. These are among the dangerous threats in the Labyrinth, as they are specifically designed to hunt beings like yourself within a foreign mind. Everyone has Censor-Beasts inside their head, roving around to remove astral parasites or dark influencers. People sucked into another's mindscape count as foreign bodies in their heads, and the Censors have the innate ability to sniff you out. You must make a successful stealth check each time you encounter them, even if you defeat them in combat, and on a failure the alarm is raised and the person's whose mind you are within realizes something is wrong. This is another reason why delving a person's mind is much easier if they are willing to the procedure or otherwise knocked out so Censors don't awaken them to your plot.
Censor Beasts (2+2 HD, +3 to hit, Bite at 1d8, Erasure at 1d4, tracking, howl)
Numbers: 2d3 in Labyrinth, 1d2 in Outer Mindscape, 2d8 near Labyrinth Center
The Censors are powerful psychically driven beings who appear as the individual's mind would perceive a ruthless, tireless hunter. For the vast majority of people, this would be a hunting dog or group of wolves, hence the name, but just as often they are monstrous looking city guards or patrolling orc justice-makers. Censors Beasts have two attacks; a “bite” which could also just as easily be a sword slash or short fire breath attack based on their manifestation, and their Erasure. Erasure only works on foes who stand still long enough for it to erase part of their being, and as such highly Dexterous fighters (+2 or better) avoid the erasure entirely.
Being erased is a lot worse then being harmed. If your character is actually physically present within another being's mind, such as through a portal or powerful spell, then when the Erasure move deals 4 damage on a hit, they take a level in level drain. Part of their being is deleted and they look and feel slightly transparent and wraith-like, which gets worse the more they are drained. If your character is not physically present in the Labyrinth, and are invading through magical and psychic manifestations or are otherwise using a proxy, then the Erasure beam deals 2d6 damage to their avatar instead.
Finally; Censor-Beasts are hunters. They exist to destroy rogue bodies like yourself within the very mind you are plumbing. They can track like dogs can by scent (though in this case, it's thought). The only way to avoid being tracked is to meditate and move without thought or leaving your 'mark' on anything mentally. Don't describe or interact with the environment. You'd have to move without mapping in order for you to lose a patrol of these beasts, which means you'll also be getting lost. Finally, Censor Beasts will 'howl' upon the death of their last, which forces another wandering monster check. If it succeeds, then the wandering monster encountered will always be another pack of Censor Beasts. In the unlikely event these Censor Beasts flee combat, then they don't howl and instead fall back to dissipate into the mind.
 Secrets. Private memories, dirt, shameful things, and so on found in the Labyrinth. Sometimes found in memory snippets, sometimes find in abstract locked rooms or dusty corners, sometimes manifested as the prize to win in a highly defended “dungeon”. Can be used as blackmail against the person whose mind and privacy you are invading. Doesn't require anything special, just finding it and remembering it is is enough to learn their dirty secrets.
 Skill. The “well of experience” appears as a literal well with shimmering gold liquid. It may also appear as small piles of coins (such as coins won in successful duels) or little portraits with moments of success from the labyrinth owner's life. This is their abstract experience. Each “load” unit you find and take is equal to 500 experience points, which is lost from the owner and given to whoever absorbs it first. Stealing from monsters/NPCs will instead cause a lose in HD and related abilities. Characters who are in line with the archetype get the full experience value, but characters who are on a different path may only receive half experience as it doesn't help them learn as well.
Note that as an abstract item, it may not manifest in the real world correctly and may simply dissipate like a dream when you leave the Labyrinth, unless you have a special container that can keep something abstract like an idea in a physical form.
 Abilities. Similar to skills above, but this could be about entire specific abilities the person has instead. Magically or psychically you could take out a person's ability to speak a certain language, to pick locks, a blade art, or a specific spell or group of spells. When taken out of the person's mind, these will manifest as an item that can be worn to grant that ability to somebody else. Spells turn up as scrolls or can be transcribed on scrolls; which can then be cast or used to write in a spellbook as normal. While not totally realistic or in-line with the fiction, it may be possible to do things like steal a snake person's ability to create venom and forge it into a vial that refills itself with venom every new moon instead. Stealing or trading innate powers into magical items.
 Friends. The minds of people contain imaginary friends from their childhood, half forgotten faces of people they haven't seen for years, and even passengers and beings from the astral planes or shared through thoughts. While most of these beings will have a great deal of loyalty, even love, for the progenitor of the Labyrinth, they may also see a chance to leave as beneficial. Outside of a person's mind though, these beings are either like living illusions or ghosts without bodies in the physical world, but inviting them into your own mind could act a bit like a helpful custodian within yourself, acting as an extra guardian. In these cases, you can always speak to this being while its inside your own head just with a thought, letting you gain advice or companionship. Even while physically alone.
 Identities. Using the magic of names, or some kind of super Rogue-Disguise power, stealing the “faces” or “names “ from within the mind of a person could allow one to disguise themselves in real life. After all, the mask made from the memory of a dead man will pass much better then a second or third hand retelling of the man's appearance. Also speaking magically, stealing some bits of a person's mind could be a powerful method to create “proxies” of them; voodoo dolls weaved from cloth found in a person's own mind could create a very powerful method to control them or protect yourself from their arcane witchdoctory.
 Actual treasure. While uncommon, it is possible for certain things inside people's minds to be valuable in and of themselves. For example, a rush of positive emotions or a feeling of comfort could be taken from a mind and given to someone else for a payment. Certain creatures, like some fantasy versions of dwarves or nonhumans in general, may not be capable of “true” creativity and as such stealing the creative spark from inside a human brain will allow that being to become truly creative, but of course robbing these qualities from your first subject makes them incapable of them and leaves them permanently decreased in their quality of life. There is also always the chance of a fantasy wizard storing something valuable or like a pile of gold in someone's subconscious; this is why after a particularly good dream they find a gold coin on their pillow when they wake up. Must have slipped out of their ear.