D20 3.root(-36): Rules: Magic

Of course, no fantasy setting would be complete without magic. And people able to wield magic is not universal, but it is one of the standard elements. Settings without their Gandalfs and their Dumbledores are far less common than those with them.
One of the core ideas here is that of skill-based magic. That is, the more skilled someone is at a mundane ability, the more skilled they are at its magical equivalent. For this to work, of course, magic can't simply be automatic; it has to have a better chance of working the more skilled the caster is, so the default chance can't be 100%.
Another idea is that, rather than using Vancian magic (each caster has a set number of spells per day; each spell has a level and binds a slot of that level), spells in this system instead tire their casters and potentially harm them. A spell can, using some items from the core system, be overcharged, inflicting more damage to its caster in order to get a stronger effect if the spell works.
The third balancing factor in this system is that spellcasters have fewer skill points and fewer feats. In other words, a spellcaster can do more things with a given skill, but a mundane is better at those skills.

Some characters can learn to cast spells. In order to cast a spell, a character must know the spell in question and must roll a skill check.
Knowing the Spell: Some characters have a "Spellcasting" class feature. Others may have a racial ability that grants them a spell or a feat that does the same. If a character has learned a spell or has something in an ability description stating that they know a spell, they may cast it.
The Skill Check: When casting a spell, a character makes a skill check using the highest of the "Casting Skills" listed in the spell description. Each spell is considered a use of the skill, with the following modifications:

• No auxiliary skills. No ability or skill other than the one used in the casting roll may be applied to the check.
• No magical bonuses. No spell worked upon the caster or the caster's equipment may grant a bonus to the check or negate a penalty. Even spells that directly enhance the caster's own abilities or skills have no effect on the casting roll, unless they specifically say otherwise.
• Common traits. All spells have the following lines in their descriptions:
  

Trained Only: Yes.
Contradiction:  No.
Failure Margin: The improperly handled energies backlash on the caster, inflicting 1d2 damage. Some spells may have other effects that may be used instead of this one.
Margin Rate: 3
Success Margin: Increase the DC of this spell's saving throw by 5. All spells have other effects that may be substituted for this one.
Try Again: Yes. If the spell simply fails, the energies dissipate immediately; if it succeeds, the energies are bound into the form the caster wanted and do not affect other spells unless the spell's description says otherwise.
Armor Check Penalty: Yes.

Casting Fatigue: After the casting roll, the caster begins to wear out. For every 5 points of DC, fatigue the caster by one factor.
Saving Throw: Most spells allow a saving throw. The target of the spell attempts a save (DC 10+casting ability+margins of success). If the save fails, the spell has its full effect. If the save is passed, some other effect happens:

• Negates: The spell has no effect on this target.
  
• Half Effect: All numerical effects of the spell are halved.
  
• Half Duration: The spell duration is halved.
  
• Partial: The spell has some alternate effect.
  
• Reverses: The spell is treated as though it had been cast on its caster by this target, with no save permitted.