Dream Magic

History

For a long time, it was believed that the only place that any form of dream magic could be practiced was on the Dreamscape and usually with the help of certain items that are native to the Dreamscape such as Dreamglass. After the Dreamscape was locked, it was presumed that dream magic was lost as well.
The first people who discovered that dream magic could be used anywhere were the members of The Cult of the Void. Though their records are difficult to decipher and written in their custom language, they are the only records we have access to that says how dream magic works.

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Mechanics Overview

At its core, dream magic is the ability to make changes in the fundamental fabric of the observable world through the application of one's own will and imagination. It is a highly freeform practice with the only limits being imposed by the user's mental strength versus the mental strength of any other dream magic users who attempt to counter the effect.
But in exchange for this freedom, dream magic takes a toll on the user's sanity as it requires prolonged acknowledgement of the unreality of the world we live in.
This price can be avoided by using items made from dreamglass for that purpose, but it also requires placing strict limits on the user's abilities and tying the effect to something that the user already understands.
For those who eschew these constraints and use dream magic without items, there are three main categories of possible effects, with each requiring application of a different mental process.

Changes to the User

Typically, the easiest changes to make are to the user themselves, with mental changes being easier than physical changes. These are also the hardest to counter by other users. However, they also have the highest sanity cost (1d10 on first use, with each subsequent use going down by 2 until 1d4).
Examples of dream magic changes to the user include making themselves forget an event, giving themself resistance to certain types of damage, growing spikes on their body, or even making themselves immortal. Any of these required unwavering concentration on the idea without allowing doubt to enter their mind (aka Constitution check).
A user can make this easier by tying it to something the user already understands. For example, it is very hard simply believe you are immortal, but it is easier to believe that you are immortal because your soul will leave your body and inhabit an item when you die. The former is something that not even gods can accomplish, but ghosts and hauntings are relatively common in Faerun.

Changes Outside the User

Because dream magic fiddles with reality itself, any changes that a user can imagine are theoretically possible with the application of a sufficient amount of willpower. The user's chances of success increase if the user has a full understanding of the properties of the object they are starting with and the object they want to change it into (aka Intelligence check).
However, once the user's mind attempts to influence something that everyone can see and experience, then the user must take everyone else's mind into account as well (aka Wisdom saving throws). It isn't very useful to change a pen into a sword, for instance, if the sword can't cut anyone.
The chances of the user's changes being effective and permanent depend largely on how convincingly they make the change and whether someone watches the change occur. Most people will assume that such a feat is only an illusion, and if that belief is stronger than the user's belief that it is real, then the stronger belief becomes reality.
Changing discrete inanimate objects into other discrete inanimate objects is the easiest type of change that a dream magic user can do. It becomes easier if the user makes sure that the starting object has some properties in common with the final product, such as being made of the same material, having the same shape, or being the same size.
Next in difficulty is changing a physical object into an abstract concept. It is possible to transform something like a rock into the feeling of love, but such effect tends to be fleeting and difficult to direct. It is more common and useful simply to imbue the physical object with the abstract concept as if it was a magical item. In that case, the rock would make a person who holds it feel love.
Outward changes are most difficult any time they involve living creatures, whether that is changing a living creature in some way or changing an inanimate object into a living creature. Changing a living creature in particular faces stiff resistance and requires more skills in persuasion than imagination (aka Charisma check), especially if the living creature is also sapient. Furthermore, any living creature created by dream magic will have its own will and be no more likely to obey you than any other creature.

Creation

As with changing, creating inanimate objects is much easier than creating living creatures and living creatures that are created will have their own will immediately. The major difference is that it is simply much more difficult to create something from nothing than to start with an object the user already believe to be real. For this reason, this ability is used only when no disposable object is available.

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Collaboration

Unlike other forms of magic, dream magic lends itself very well to collaborating with other users. The more people who can focus on a singular goal at the same time, the easier it is to accomplish and, therefore, the bigger the goal can be.
Groups of dream magic users can either all focus on the same goal together (designate a leader for the group; each member of the group makes the relevant check and for each successful check, add +2 to the leader's check. If the leader fails, the whole endeavor fails). Or they can each focus on a piece of a large goal (each member of the group must pass their own DCs to succeed, but no one member can cause the whole project to fail).
Typically, an individual user can achieve pretty much anything if they are mentally strong enough. But very large or complicated projects, such as creating buildings or changing large landscape features, are so difficult as to be functionally impossible without others to collaborate with.

Type

Metaphysical, Arcane