Because they're so central to the game, and because most of us can't use our natural intution to know what a mage with an "average" ability can do, the magic rules will be a bit more complex than those for other abilities.
Casting Spontaneous Spells
Spontaneous spells can never exceed the following limits: They cannot affect an area larger than you can see (though IntÚllego spells may effectively increase your range of sight and effects may linger after you leave). They cannot have durations longer than a month. They cannot be used to perceive the past or future.
Casting Formulaic Spells
Regardless of the outcome, if the number of natural 10s rolled on the dice exceeds the character's willpower then roll for the possibility of Wizard's Twilight.
In addition to using vis to increase the spell roll (see below), most formulaic spells have a special built-in option for vis boosting which will increase either the range or the duration.
Formulaic spells cannot be used to perceive the past or future.
Formulaic spells can never exceed the following limits unless it is the result of vis boosting (as above): They cannot affect an area larger than you can see (though IntÚllego spells may effectively increase your range of sight and effects may linger after you leave). They cannot have durations longer than a month.
Casting Ritual Spells
For every multiple of the base time, roll as many d6s as your skill level in that spell type (if multiple casters use the average), and add them up. If the total die roll exceeds the ritual's minimum level, compare the total of the dice to the spell effect tables to determine the effect; the effects are open-ended and the casters can choose exactly how to apply extra power beyond the minimum. If the roll is below the minimum level, then it fails. Particularly low rolls can result in spectacular failures.
All spells require two-handed gestures and speaking aloud. Formulaic and ritual magic simply can't be cast without these conditions. Spontaneous magic can with the following penalties. Note that an individual die can be a negative number.
All spells require the magus's total concentration. Assign the following penalties due to distracting conditions.
In addition, if any of the following events occur while casting or maintaining a spell, then the caster must make a Willpower roll to continue, using the target values below. If the roll fails, then the spell fails or concentration is broken.
Both spontaneous and formulaic magic can be performed a little faster or slower. Subtract 1 from each die for each five seconds cut off the casting time. (Yes, spontaneous spells are instantaneous with a -2 penalty). Add 1 to each die if the casting time is tripled.
Individual Twilight episodes are highly variable, depending on the cause and the conditions surrounding it. You may remain fully conscious, retain only limited control, fall unconscious, or go into fits. They may last for seconds or days. Depending on the strength of the episode, Twilight may leave permanent marks on you, some good (increased understanding or magical affinities), some bad (physical deformities, mental aberrations, or magical flaws). You may use Willpower and Enigmatic Wisdom (a unique ability taught by House Criamon) to attempt to "control" the Twilight episode and influence toward beneficial outcomes. Whatever the outcome of an episode, it tends to leave a residue known as Twilight points. Each Twilight point is a negative ability which subtracts one die from attempts to resist (but not control) further Twilight episodes.
As the years pass and Twilight points accumulate, some magi reach a point where they enter "final Twilight", when the magus's soul leaves the worldly realm for some other place. (The body may or may not pass on with the soul.) Hermetic theory holds that final Twilight is not death, but a translation to some higher plane of being. It is a certainty that magi who pass beyond Twilight may not be summoned as ghosts or contacted with any spell.