I just came upon this, and its a good read, especially since DF will use some kind of magic system later on.
Perhaps it will be helpful for Toady also.
CREATING A MAGIC SYSTEM
In most fantasy games, you will likely wish to include some sort of magic. Including magic can be fun, but it can also be taxing. Many leave magic as a very open-ended subject, letting those who join decide upon how the magic will work for their character. It is important to clearly define how your magic system works so less confusion will surface later. I will cover Basic Magic Concepts, Spell-Casting Procedure Methods, Accessories, Limitations, and Elemental Inclusion.
(I) Basic Magic Concepts
These govern the entire origin of magical energy in your magic system. It is important to take great care and detail when choosing an area and tweaking it to your needs, because this is the basis for the your whole magic system. There are 3 main divisions of magic that I have compiled: Inner, Outer, and Object. Usually a magic system will feature a combination of these 3 to varying degrees, but the choice is all up to you.
INNER
Inner Magic deals with spells that derive their energy from the individual. Such concepts as Chakra emphasize such an inner energy. To perform a spell, one must usually focus and release their inner energy to achieve the desired effect. This source of inner energy is usually limited but can be restored through rest or nourishment. The user's physical fortitude does hinder the use of Inner Magic, because it is directly linked with physical fortitude. The basis for inner energy is upon the physical and mental/emotional energy that we use throughout every day.
OUTER
Outer Magic deals with spells that derive their energy from everything around them. Such concepts as Chi emphasize such an outer energy. To perform a spell, one must usually will and channel the outer energy to achieve the desired effect. This source of magic is usually ever-replenishing, being scarce in some areas due to many spells being cast in tandem. The user's physical fortitude will most likely hinder the use of Outer Magic, because great amounts of strength are required to command the loose/free energy. The basis for outer energy is upon how air flows through our atmosphere and is used by our bodies.
OBJECT
Object Magic deals with spells that derive their energy from artifacts that contain and radiate magical energy. To my knowledge, no existing real-world concept emphasizes such an object energy, though it is related to in a myriad of myths and the like. To perform a spell, one must somehow intelligently utilize an artifact to achieve the desired effect. This source of magic is usually limited but can be recharged by various means. The user's physical fortitude does not hinder the use of Object Magic, as it usually requires little strength. The basis for object energy might be upon radioactivity and how waves pass through and react to matter.
(II) Spell-Casting Procedure Methods
Spell-casting is usually done by some expression of the caster. While not using any is possible, these will usually be utilized efficiently. This is a list of procedures possible to perform the casting of a spell. I have thought of 3 main categories: Vocal, Manual (Tactile and Physical), and Visual.
VOCAL
Vocal casting involves recitations, invocations, incantation, and chants. The purpose is to proclaim a certain word or phrase aloud. Usually, the louder spoken, the better. The longer the phrase, the stronger the spell. While vocal casting may be said mentally, that is commonly reserved for very skilled magic users.
MANUAL
Manual casting, divided into Tactile and Manual, deals with actions.
TACTILE
Tactile casting deals with executing a sequence of hand signs and/or body positions. Usually, the clearer the hand signs and/or body positions, the better. The more steps in a chain of hand signs and/or body positions, the stronger the spell. To avoid misunderstanding, I will provide an example: "To cast a spell, one might have to squat and clasp their hands tightly."
PHYSICAL
Physical casting deals with motions, striking objects, and maybe even dancing. Usually, the more detailed the ritual, the better. The longer it takes to perform a ritual, the stronger the spell. Since this is a more confusing and vague area, I will provide an example: "To cast a spell, one might have to run in a circle around a flame while brandishing their sword mightily."
VISUAL
Visual casting encompasses symbols drawn and words written on some medium. The purpose is to relate a geometric or odd shapes to a spell. Usually, the more defined the drawing, the better. The more intricate the symbol, the more powerful the spell. While visual casting may be pictured mentally, that is commonly reserved for very skilled magic users. If using words, it could simply be a different form of Vocal casting, seeing as the same words could be either spoken or written.
(III) Spell Limitations
Almost always, roleplay creators choose to place limits upon magic through various means (if not a part of the plot or magic system, then to provide balance as a roleplay mechanic). This can be achieved in different ways. I have organized the 9 most significant limitations: Mana Cost, Cast Time, Malfunction, Disabilities, Prerequisites, Complexity, Specifics, Laws, and Principles. A good magic system will implement many of these, but there are many exceptions.
MANA COST
This is probably the most important limitation to place. Basically, from whatever form of energy your magic system draws, there has to be limit. This can be either upon the energy's remaining amount itself or upon the individual's stamina. This is usually scaled according to a spell's magnitude, duration, and range. Such, powerful, long, and far-range spells will cost much more than weak, short, and close-range spells. There should always be a cost for any spell.
CAST TIME
Another good limitation, it defines the time involved in spell-casting or preparation thereof. Cast Time can be divided into 2 areas: Warm-Up and Cool-Down. Warm-Up is the time before and during the casting of the spell (but before its effect is manifested). Cool-Down is the time after a spell's manifestation has ended before another spell may be cast. Insta-casts, spells with no cast time, have their place, but they should mainly be used with either weak spells or when other limitations satisfactorily limit the spell's use.
MALFUNCTION
This limitation is useful for unrefined and unpracticed spells. Possible Malfunction balances out powerful spells as well, because all the other limitations that go into casting the spell might be wasted if the effect does not work properly. Malfunction may also occur when certain prerequisites aren't met, or if the casting of the spell is somehow interrupted. The basic areas of malfunction are Backfire, Failure, and Diminish. Backfire means that the effects of the cast spell are redirected to you instead of the target. Alternatively, if a spell that is meant to be cast upon yourself backfires, it would redirect to another entity. Failure is simply when the effect of the cast spell doesn't happen. Diminish decreases the effect of the cast spell, reducing its magnitude, duration, or range.
DISABILITIES
This limitation covers boundaries placed when a spell has been cast. For instance, a given spell might require that the caster stay motionless for its duration. So, the caster will be unable to move. The most common use of Disability is mandating that no spell can be cast while the effects of another are still being manifested. For more potent spells, the price might even be loss of one of the 5 senses or stunted health. There are numerous other possible disabilities that can be applied to spells.
PREREQUISITES
An overlooked limitation, it governs the conditions necessary for a spell's effect to be manifested. It is often the precursor of the 2 Manual Spell-Casting Methods. As an example, a spell might require direct contact with the target; so the prerequisite limitation for casting that spell is that the caster be within contact distance of the target. Prerequisites have several other auxiliary uses during the spell-casting process.
COMPLEXITY
This limitation is either not considered often or is naturally woven into other aspects of your magic system. Regardless, I felt it needed mention here. Complexity is the gauge of the difficulty of correctly remembering and properly performing spell-casting procedure methods. While Informant Accessories can alleviate this problem somewhat, you then must keep your accessory safe, have the time to find the information within, and comprehend what is read. Complexity also affects situations where time is important, such as in mid-battle and while sneaking around. So, Complexity may also cause an increase in the regular Warm-Up Cast Time of a spell.
SPECIFICS
Occasionally, the very nature of a spell's effect poses a problem for the caster. For instance, a spell might only affect those with a certain trait. Such, nobody else would be affected, and there's nothing that can be done to change that. This is a strong limitation for spells that would otherwise be too great. It should be used but cautiously. In example, a given spell might impede genetic water-breathing ability; so those belonging to a race of amphibious people would suffer while solely land-dwellers wouldn't be affected.
LAWS
Though not a true limitation to any spell itself, it does affect their use. Laws are governmental orders and rules that pertain to the use of magic in the roleplay world. Maybe all forms of spell-casting would be illegal in certain towns. Certain groups of spells may not be allowed during the day.
PRINCIPLES
Magical principles are certain standards that a magic system follows inherently. These principles are made of different aspects of your magic system made universal. They are usually for the purpose of imposing a certain mechanic upon all members in your roleplay. These is more important for the more obscure magic in your system, like abjuration, transmutation, and apothecary.
(IV) Elemental Inclusion
Including elements in a magic system is vastly popular, but remember to keep balance in check. A majority of these will feature the 4 classical western elements of air, water, earth, and fire. Naturally, no elemental system is restricted to these. There are a nearly infinite number of variations and possibilities such as electricity, ice, vapor, nature, sand, lava, metal, and sound. I purposely chose to not have any magic types relating to pure, light, or dark magic listed, because I feel that they aren't the same as other elements (concerning balance, especially cyclic harmony). Whatever the choice of elements, there are 4 serious considerations: Environmental Reliance, Cyclic Harmony, Tolerances, and Affinities.
ENVIRONMENTAL RELIANCE
A field that is nearly always left vague is Environment Reliance. Environmental Reliance is simply the need for a pre-existing elemental source. The two choices are either Synthesis or Terra. Synthesis is independent of environmental conditions, while Terra is reliant upon the sate of the environment. Air- and Earth-based magic usually don't encounter the problem of environmental reliance, but water- and fire-based magic need the clarification. Since what I've said so far isn't terribly clear, I will give an example. If your magic system follows synthesis, a fire spell will work no matter where it is used (unless underwater, of course). However, if your magic system follows terra, a fire spell will need to draw from an existing substantial source of flame.
CYCLIC HARMONY
Cyclic Harmony maintains a balance of elements in a magic system. To put it simply, for every element, one element dominates it and another element is dominated by it. Often increased or decreased damage is associated with the relationships in the cycle. When arranged, they make a cycle of superiority and inferiority. If there is an even number of elements in the cycle, every element will have its polar opposite.
TOLERANCES
Tolerances are a way of making a character resistant against one element and vulnerable against its opposite (it must be proportional to be fair). Naturally, having more protection against an element (like thick clothing against cold magic) isn't the same as a true elemental tolerance. Choosing to make characters equally tolerant of all elements is fine, as it is still equal to utilizing tolerances. Whether or not to give your players the option of elemental tolerances is a tough choice.
AFFINITIES
These are quite similar to Resistances, but they deal with casting, not receiving, spells. Affinities are a way of making a character exceptionally talented at one element and extremely horrible at its opposite (it must be proportional to be fair). Obviously, a character using an element more frequently (because they happen to know more spells of that element) doesn't define an affinity. Choosing to make characters equally talented at all elements is fine, as it is still equal to utilizing affinities. Whether or not to give your players the option of elemental affinities is a tough choice.