Most of these ideas below are stolen from Robert Jordan's Wheel of Time series.
This is the realm beyond the veil, the source of magic, where spirits and Gods dwell, where souls go before the Psychopomps come to collect them. It is a place where thought and action are one and the same, where emotion controls the weather and ideology forms the land upon which people stand.
In Nukaria, it is referred to by various names such as the Spirit World, the World Beyond, the Plane of Thought, the Land of Dreams, The Void, The Emptiness, The Floating Realm or Deep Heaven but the most common among Magi is the Astral Sea.
This is the lowest level of the Astral Sea, or perhaps the highest. This area directly borders the material world. The Ethereal Plane is usually what it is referred to by the uneducated who regard it as another plane and not merely an extension of a higher one. Spirits and smaller spiritual entities have the ability to step between this world and our world at will. Sight Beyond Sight allows a creature to peer into this world, which is how they see creatures as they truly are, for they see the shape and nature of their souls.
The Shallows naturally resemble our world. It is a mirror of our world, with a few exceptions. Firstly, there are few, if any animals. Only animals intelligent or powerful enough to reach the Astral Sea will appear here. Some species have the ability to enter the Shallows when they sleep, such as Wolves and Dragons. Secondly, structures erected by mortals will appear where they should in our world. The longer something has existed, the more likely it will be there.
For instance, a house that has stood for many years will appear where it was built in the mortal world, while a tent erected a few minutes ago will not. Things that have been there for less than a day will appear ghostly and insubstantial. Things that are often changed will shift back and forth. For example, the contents of most people's desks will shift around and move as you watch it. Papers will disappear as you read them, laptops will shift between open and closed, mugs will fill and empty while teleporting around the desk.
People's clothes will also change, depending on their moods and current thoughts. The more experienced someone is, the less things will change on their body. For example, if a woman is thinking of her lover while she wanders here, she might find herself wearing a low-cut dress that compliments her figure, while if she was worried about being spotted, she might find herself wearing all black with a mask to conceal her face. If she was worried about being attacked, she might find herself in armor, carrying a sword.
By contrast, a more experienced traveler's clothing will change much less often, or not at all. Their clothing might even appear finer, freshly pressed, or their jewelry might shine more than possible in the physical world, but otherwise they will likely maintain a stable image in this world.
Additionally, these changes to a person's clothing and physical appearance will mostly occur only when the creature is here in Spirit. Those who come in the Flesh find much less extreme changes await them, or few at all. That being said, they can still change, or be changed as a creature here in Spirit can. But more on that later.
While in the Shallows, it is also possible to observe the physical world that you left through reflective surfaces such as mirrors, polished metal, puddles of water or anything else that can reflect light. This is why Magi fear mirrors and never keep any around them, for they know what might be watching from the other side.
Wading in the Shallows:
If you travel to this world, you can go in two ways, in Spirit and in the Flesh.
In Spirit- Mortal souls naturally dream, but occasionally even ordinary dreamers can wander into the Shallows. They usually remain for only a brief time before disappearing. Do you remember ever having a dream where you were falling or another one that was shockingly realistic? Odds are you appeared in the Shallows briefly. Luckily, your mind slipped out of there, falling back into ordinary sleep.
This is fortunate for you, as any injuries sustained in the Spirit World appear on the person's body. Dying in the Spirit World will not cause you to wake up, but will leave your soul trapped there till the Psychopomps come to collect you. It is as real a death as any other.
Spending time in the Spirit World in Spirit is not nourishing to the body like real sleep and is as strenuous as any other physical activity.
There are multiple ways to enter the Spirit World in spirit. Certain hallucinogenic drugs and magical herbs allow a dreamer to travel there and remain until they wake up. There are magical artifacts that allow the untrained and unskilled to enter. There are meditation techniques that allow someone disciplined enough to detach their consciousness and travel here. And there are spells that allow a caster to move his or other's spirits here.
Coming here in the Flesh is in some ways, more challenging, but easier in other ways. Firstly, powerful magicks can allow a creature to come here in the Flesh through methods similar to teleportation or planar travel spells. Secondly, there are natural points of entrance that, if certain rituals are completed, allow a creature to enter through a variety of natural portals.
Astral Combat and Astral Clay:
There is another trait of the Shallows that I should mention. While here, the world functions according to the normal laws. People still walk on the ground, breathe air, sink in water, etc. Except, they don't. Not really. Their minds think these things are how things should be and so, they are. But it doesn't have to be that way.
By making a check, someone in the Shallows can enforce their will on the place, altering it and transforming it in whatever way they wish. These changes rarely last, but on a success, they become real, if only for a moment. These checks use whatever mental ability score governs a character's will. One can make a check on their turn as a free action, in conjunction with doing something else.
Generally, the bigger the change, the harder the check. If another creature opposes the check, the creature should also roll and the higher result wins.
Robert Stronghand and Jennifer Goldheart are fighting in the Shallows. Robert fires an arrow at Jennifer from his bow. Jennifer, instead of dodging, thinks- no, she believes, that her cloak is hard as steel. Thus, the arrows bounce off her cloak, doing no damage.
Jennifer then chases off Robert into the forest. Robert believes in his heart "I am invisible" and hides behind a tree. Jennifer bursts through the bush, but doesn't see him, because he actually has turned invisible.
There are other skills that the Shallows can grant someone, should they know how. Those experienced with the Spirit World can move rapidly across the landscape or teleport to any location they know enough about. They cannot teleport to a place they have never been, though they might be able to get close. For example, if you have never been to New York City, you cannot go there. But you might be able to teleport into New York State, or at least to New England.
Creatures can also create things from thin air in the Shallows, though the more complex and unrealistic something is, the harder it is to create. Additionally, when something is created, that thing stops existing the second the traveler stops thinking of it. For example, creating a castle of crystal and glass would be very hard, but creating a stand mirror to look at yourself would be fairly easy to someone who knew what they were doing.
The one exception to this rule is that of living creatures. If a dreamer or astral traveler creates a living creature, that creature will gain a semblance of life and will. These creatures must be overpowered and forced back to non-existence. Furthermore, even if such a creature is designed with certain parameters, the longer it is left alive, the more it will begin to change. Since the Astral Sea is essentially a soup of pure consciousness, the longer these artificial creatures exist here, the more likely they will become conscious and stray from the template established by their creator.
And this is where the advantage of coming to the Spirit World in the Flesh comes in. Creatures there in the flesh add double the relevant modifier to any roll made to alter the Spirit World in this way- in my game, this is COG, but in yours it might be INT, or CHA. For example, if you have a COG of 13(+1), then you add +2 to any roll to mold the Astral Clay if you come here in the Flesh.
Creatures who come here in the Flesh also have advantage on their rolls when contesting any creature not here in the Flesh.
Traveling deeper into the Astral Sea involves flying up into the air, into the sky. Leaving the Shallows behind, one will enter a vast, inky space full of glittering lights that resemble stars. But these are not stars, but the dreams of all creatures currently dreaming, in this or any other world. Experienced Spirit World travelers and dream-walkers can zip through this dark space and find the dreams of a particular individual.
They can then enter that person's dreams, though this is hazardous.
While inside someone's dream, they are in almost total control. They can do whatever they want to you, though they might not be fully aware that it's you in there. They probably also aren't in control of their dreams. It is difficult for a dream-walker or traveler who ends up in someone's dream to escape if the dreamer wants to keep them there, whether intentionally or unintentionally. If the dreamer is holding them there, they can only leave when that person lets them go, or if that person wakes up.
Some people, gifted with the ability to control their own dreams, but not enter the Shallows, can hold entire conversations with a dream-walker and remember whatever happens come morning. This can be an asset to many dream-walkers, who find such people to be refreshing to speak to, as it is very difficult to communicate with normal people through dreams. There are some creatures who can pull an ordinary dreamer with them into the Shallows, but most such creatures are malicious and only seek to harm the dreamer in question.
No harm, or at least, no physical harm can come to you in someone else's dream.
A dream-walker is peering in on the dreams of a man she knows. She enters his dream and finds him riding a horse across the beach near his house. While she doesn't know it yet, this man has a crush on her. So when he sees her in the dream, she will probably become more beautiful and depending on his inclination, wearing something better looking, sexier or maybe nothing at all.
Hopefully she's okay with him seeing her like that, or if she's not, hopefully she can escape his dream, otherwise it's going to make their interactions very awkward for her in the future. Remember, she will remember the dream, but he might not.
The Dreams of the Divine:
But there are dreams, and then there are Dreams:
In the Depths, certain powerful entities make their dwellings here. These entities go to sleep and create vast dreams which can hold not only them, but many countless others. Inside, these dreams can be large and detailed enough to contain cities, nations or even entire worlds. It is here, in the dreams of the Gods, that you can find the Upper Realms from where Outsiders that we name Angels and Demons truly hail from.
These worlds often resemble cities or nations in their most superficial ways, but are clearly designed for creatures not familiar with things such as three spatial dimensions or linear time. Gravity is often a suggestion, where it exists at all. Buildings flow and shift, changing with the whims of those who dwell within them. Bizarre and unnatural weather occurs constantly, though here the abnormal is normal. The creatures here are strange, ever-shifting beings that adopt new forms as fashion, custom and whim changes. Not all will be stronger than you, but their will be those who wield great power. Others will be weak, little more powerful than the small beasts or insects in your own world.
Depending on what Dream you enter, these creatures might greet you warmly. You might find this a land populated by friendly, if alien beings, or one frequented by travelers from other worlds. Others are unforgiving and completely inhospitable to creatures such as you, even bordering on the incomprehensible as objects and creatures pop into existence around you before just as swiftly disappearing, or events occurring out of order, or linear time simply not existing here. Even the most experienced of planar traveler will likely find the Dreams found in the Depths of the Astral Sea to be unpleasant, confusing or downright dangerous.
The afterlives of various Gods belonging to certain exclusive cults can be found here.
Renegade entities often hide here as well, for the nigh-infinite space in the Depths means that even the most persistent of hunters will find it difficult, if not impossible, to find them here. Here they can celebrate and do as they wish without interference. Grazui, Beloved Master of the Kytons, makes his realm here, as do many of the Princes of Chaos. There are likely other creatures who lurk down here, in the darkness beyond reality. Who know what lurks down there, in the blackness that never ends?