Plane: The Dreamlands
The Astral Plane is known for good reason as the Realm of Thought. It is the source of all magic, the base from which the deities have formed their realms: a place formed from the incredible power of the collective unconscious of every thinking creature in the multiverse. It should come as no surprise, then, that the dreams of living creatures are more than figments replayed by their brains. In the Astral Plane, they take on a very real form.
The demiplane of the Astral in which dreams reside is colloquially known as the Dreamlands.
Types of Visitor
There are three types of visitors to the Dreamlands: lay dreamers, lucid dreamers and interlopers.
Lay dreamers are those whose physical forms are peacefully slumbering somewhere in the multiverse. They reside almost exclusively within their own dreams, and are rarely aware that they are even dreaming. If something happens to make them aware of this fact, they must immediately make a saving throw vs. death. If they fail, they immediately awaken. Otherwise, they become lucid dreamers until they next awaken.
Lay dreamers control the dreams in which they exist, even if they do so subconsciously. Even if the dream in question is a horrific nightmare, it is their nightmare. Their mind will not allow them to be killed in their own dream, and even any harm done to them by it is purely an illusion. If some harm is done to them by something foreign to their dream - like a malicious visitor - their subconscious will instantly register that something is wrong and they will either wake up or become lucid. Either is equally dangerous to the one who did them harm, so dream assassinations are usually not well advised.
A lucid dreamer is anyone who is aware they are currently dreaming. There are a number of ways in which this might happen. One, as noted above, might happen temporarily when a lay dreamer is exposed to a shock. There are also magical means, such as certain rituals or the 5th-level wizard spell Dream. Finally, lucid dreaming is a skill that can be learned. Specifically, it is a Non-Weapon Proficiency (Wizard/Priest, WIS-4, 2 slots) that allows one to enter a lucid dream each night a successful check is made. However, hours spent lucid dreaming are not restful and do not restore hit points or allow spells to be memorised.
Being a lucid dreamer changes things significantly. Lucid dreamers begin in their own dream, just like lay dreamers. They have complete control over it, however. If an outsider enters the dream of a lucid dreamer, they are in deadly peril the moment the dreamer becomes aware of their presence. Within their own dream, a lucid dreamer can do, become, or cause to happen anything they desire. They can transform into a dragon, cast spells beyond their ability (even if they are not spellcasters), or make a horde of demons stream from the ground. They are all-powerful - but only within their dream.
An interloper is someone who does not belong in the Dreamlands. They have arrived by astral projection, plane shifting, or physical travel from the Astral Plane. They are in the most dangerous position of all, since they have no power to control the dreams around them and everything in the Dreamlands is completely and terrifyingly real to them. They do not even have the luxury that lucid dreamers have, of instantaneous travel - just getting from one dream to another can be a great trial with many dangers.
Lucid dreamers can leave their own dreams and travel to other parts of the Dreamlands as long as they have the Dreaming NWP, although they are at risk once they do so. They are no longer omnipotent outside of their own dream, and can be harmed. Every hit point of damage deals a single point of sanity damage to their psyche, which must be restored via the normal means.
They cannot change their physical shape outside of their dream; it is a representation of their subconscious (though druids and other shapeshifters can assume their other forms at-will). They can, however, change their clothes and equipment. Inexperienced dreamers often change randomly depending on their thoughts and mood - if they are about to enter a dangerous dream, a suit of armor might appear around them. With a successful Dreaming check, you can create and maintain whatever clothes and equipment you desire - though you cannot make magic items. Only certain specific enchantments designed for the purpose can be carried into the Dreamlands with you.
In terms of combat, lucid dreamers function much the same in the Dreamlands as they do on the Material, once they leave their dream. Any skill they have with weapons remains available to them; furthermore, they may cast spells normally. If you have cast your one fireball for the day, however, it will not be available in the Dreamlands. Likewise, spells or abilities used up in the Dreamlands are gone when you awaken - you are not dreaming of using them, you actually are.
Lucid dreamers with the NWP can travel about instantly to any point within the Dreamlands. Upon leaving their dream, they will find themselves floating in the Reflected Void outside their dream, with the Firmament below them and the points of light that represent the dreams of others all around them. They may will themselves to any point in the Firmament they are familiar with, as well as into any of the dreams they can see. If they have visited a particular person's dream before, they can always find it again. If they want to find a particular person they haven't visited before, they must know them well and make a Dreaming check. If they know them only casually, it will incur a -4 penalty. If this check is failed, it cannot be attempted again this night. Of course, this check will always fail if the person in question is not asleep.
When a lucid dreamer enters the dream of another, it can be anywhere from perfectly safe to murderously dangerous. Entering the dream of a lay dreamer is usually safe as long as you don't interfere or do anything to make them realise they're dreaming, and you can leave at any time. If their dream includes you, however, you will take the place of their facsimile of you. You must make a Dreaming check to be able to do anything of your own free will (including leaving the dream) rather than simply acting as they dream you. If you fail, you are trapped until they awaken, stop dreaming of you, or until an hour passes, when you can make the check again. If they are having a nightmare, you must make a Dreaming check to leave their dream. Otherwise you are trapped there until they awaken, the nightmare ends, or an hour passes.
Another lucid dreamer's dream is very risky, for obvious reasons. As they have complete control over their dream, there is very little you can do to resist them once they become aware of you. Furthermore, you cannot easily leave a lucid dreamer's dream unless they let you. It requires an opposed Dreaming check to escape, and if you fail you are trapped until the dreamer releases you or one of you awakens.
Within the Astral, which is infinite in every direction, one travels by thought. Only those new and uninitiated would attempt to travel by mundane physical means in that endless grey void. One can not only travel at immense speeds in any direction simply by willing themselves to do so: they can also will themselves towards a destination. Thinking of a destination will carry you to it, even if you know nothing more about it than its name.
As one approaches the vicinity of the Dreamlands, the first thing they will notice is an abundance of fanciful creatures. These are the remnants of dreams that have slipped out. For the most part, dreams rapidly decay and crumble once they are no longer being dreamed. The denizens of those dreams do so as well. After all, a dream bugbear isn't an actual bugbear - it is what's known as a facsimile, the dreamer's impression of a bugbear with no true mind of its own. In some rare cases, however, facsimiles manage to survive long enough to gain some measure of sentience of their own and slip out of the Dreamlands.
Depending on just what you encounter, the rogue facsimiles that inhabit the outskirts of the Dreamlands might be beautiful, bizarre or downright terrifying. Like everything in the Dreamlands, they are very real despite their ephemeral origins. A rogue facsimile of a dragon breathes fire that is just as deadly to a physical traveller in the Astral as any danger on the Material.
If a visitor were to enter the Dreamlands from the outside, this is the first place they would find themselves. The Firmament forms the foundation of the Dreamlands, and is a representation of the "real world" - in this context, the real world is anywhere inhabited by enough thinking creatures to have a dream representation. Uninhabited or unknown areas of the world do not exist in the Firmament, and neither do the Outer Planes. As one might expect, a visitor from a specific world will tend to begin in the part of the Firmament that represents their world - and, if applicable, their current location within that world.
Unlike an individual's dream, the Firmament is shared - if you were to enter the part of the Firmament that represents Solamnus, you could well meet another dreamer there. Generally speaking, however, the Firmament tends to be rather empty - though there is always the danger of a rogue facsimile. Ordinary dreamers only rarely visit it, and usually by accident. It does, however, have some interesting properties. For one thing, the Firmament is constantly changing as the perception of those who experience it changes. Things which have been around for a long time tend to be fairly stable - the royal palace of Solamnus, for example. Things which have no fixed position or place tend to appear and disappear randomly - a barrel that is moved around a warehouse, wagons that travel constantly throughout the city, and so on.
This property can often be useful, however. Take a wizard's private drawer for keeping letters and missives, for example. In the waking world, it might be entirely inaccessible to you. In the dream world, you could pull it open - though it would return to a closed position the moment you stopped paying attention. You'd find it empty - or filled to the bursting with letters - or containing only a few scraps of paper. The more something changes, the more volatile it is in the Firmament. This feature of the Firmament, however, may allow you to read random letters that the mage had received over the past couple weeks - though only a few seconds at a time before they disappeared from your hand and the drawer returned to its previous state.
The Reflected Void
One feature of the Firmament is that it is always night there. Above, the night sky is clearer than it ever would be on the Material - and the multitude of twinkling stars above do not resemble any constellation recorded on any Material world. This is because you are looking not at stars, but at the twinkling shapes of individual dreams far above you. For an interloper who leaves the Firmament, or a lucid dreamer who has just stepped outside of their own dream, the Reflected Void is where they find themselves.
Lucid dreamers spend little time in the Reflected Void - usually just long enough to get their bearings after stepping out of their own dream. They can travel wherever they desire within the Dreamlands in the blink of an eye, so they need not worry about the vast distances between dreams. For interlopers, however, the Reflected Void represents a significant barrier between themselves and the dreams of others. This is compounded by the fact that rogue facsimiles are more common here than anywhere else.
The innumerable points of light that hang like twinkling gems in the Reflected Void are the dreams themselves. Each and every one of those points is the dream of a sapient creature that is currently asleep somewhere in the multiverse - though they may not always be "home", in the case of a lucid dreamer. As described above, both lay and lucid dreamers have control over their dreams, even though lay dreamers are themselves at the mercy of their subconscious mind.
The general rules for interacting with dreams are given above, in the section on lucid dreaming. Dreams can be very strange places, difficult to get your bearings in - especially the dreams of others. They tend not to be much bigger than the areas the dreamer has envisaged, although areas that the dreamer has left tend to hang around until the dream ends. They are also prone to large gaps; a dreamer might go from his bedroom to the town square in that absentminded manner that dreams have. For someone following him, they would find the door to his bedroom disappears into mist, through which the other parts of the dream can be seen - including the town square.
It's important to remember at all times that dreams are coloured by the dreamer. The royal palace of Solamnus is the royal palace as they imagine or remember it, so you can't assume that secret door you remember will be there. Likewise, a dream orc is an orc as the dreamer imagines it - in a child's dream, it might be 12 feet tall and terrifyingly strong.