Casting Magical Effects

Rules For Magick
Compared to many game systems, M20 has an extremely freeform approach to casting magic. More than anything, its dependent upon the players' wits and imagination. Good roleplaying is as important as dice mechanics. The foundation for using magic is to answer three questions before making an rolls: From there, it will be time to determine what the difficulty is based on adding and subtracting dice modifiers, including spending Quintessence and if you are going to spend a point of Willpower to give yourself one automatic success. For a more detailed version of this, see the Casting Magickal Effects on p. 501 in the M20 Core book.
 * 1) What does your character want to do?
 * 2) Given your character's focus and spheres/paths, what Effect can you create?
 * 3) What would any potential witness (Sleeper or otherwise) see them do? (This helps establish if the effect is vulgar or coincidental)
 * 1) Establish what type of action you are making (extended actions for rituals, etc.)
 * 2) roll one die for every dot in your Arete Trait
 * 3) Determine if the Effect succeeds, fails, or botches. Any countermagick rolls would be made at this time.
 * 4) Allow the Storyteller to describe the outcome, including how much Paradox was gained and if the character experiences the start of Backlash.

Optional Rule: Automatic Effects
There might be times when a Storyteller might let a character perform simple, non combative spells without an action roll. This option requires that the character's Arete Trait is equal or higher to the Base Casting Roll- A mage with an Arete of 4 would be able to automatically perform a Coincidental Effect using one of more rank 1 spheres.

Optional Rule: The Domino Effect
If a Storyteller feels like the number of coincidental effects thrown in quick succession are causing events to lose their plausibility, they can bring the Domino Effect into play: for every two unlikely or attention-drawing coincidences within a single scene, the ST may add a +1 difficulty to subsequent coincidences, stacking it to +2 after five mind-boggling effects, +3 after seven, etc. Subtle personal effects that no one notices (sensory enhancements, minor Trait boosts, etc.,) should not be counted towards the Domino Effect.

Magickal Difficulty Modifiers
The maximum difficulty for a roll is 10. If, however, the combination of highest Sphere + vulgarity modifier + other modifiers (max +3) reaches higher than 11+, then each +1 modifier over 10 transfers instead to requiring an additional success instead.

Optional Rule: Thresholds
Storytellers can choose to allow players to make extended rolls for Effects they would probably otherwise fail, or to reflect a task that simply involves a lot of work. They reduce the difficulty but increase the total number of successes required for success. For every level of a threshold, one additional success is needed. The highest difficulty a threshold roll has is 9 rather than 10, but it could require up to 5 additional successes. Combined with the Magical Feats system, this could make for a very long extended dice roll.

Complicated Effects (aka Magical Feats)
For effects that are more ambitious and complex, requiring rituals (aka Extended dice rolls).
 * Simple Personal Effects (changes to self) typically require only one success
 * Standard General Effects (changes to someone or something else)require at least two successes
 * World-altering Effects require at bare minimum 20 successes and probably more.
 * The maximum number of rolls you can make during your ritual is equal to your permanent Willpower Trait + your Arete.
 * The difficulty for any stamina checks or mundane activities taking place during a ritual is the ritual's base difficulty: highest sphere + modifier for coincidental or vulgar magic

Degrees of Success

 * Botch: Character makes a critical mistake, failing the Effect and all associated actions in a dramatic way.  Paradox is gained based on level of highest Sphere and vulgarity of effect.
 * Total Failure: No success; effort fails short.  Mage can try again if time is available at +1 difficulty.
 * Partial success: Player scores at least 50% of the suggested successes, giving imperfect results.  Mage can continue to try at +1 difficulty if time allows.
 * Success: Player scores 100% of the suggested successes.  Effect goes off as planned.
 * Extraordinary Success: Player scores at least three successes above the suggest successes, providing an appropriate bonus to the Effect to be decided by the Storyteller based on the nature of the Effect.

Base Damage/Healing or Duration or Targets
The  base = number of successes required for the type of Effect. Personal effects, simple self targeted spells require just 1 success, for example but an Effect to affect someone or something else requires at least 2 successes. To preserve game balance, the ST may choose to cap damage at at 20 health levels (10 successes or 9 successes for Force Sphere attacks which add 1 automatic success to Damage.
 * Bashing Damage: Mind Sphere Effects
 * Lethal: Most other Sphere Effects (Time and Correspondence inflict no damage unless combined with other Spheres and Entropy only inflicts damage through indirect acts until rank 4)
 * Aggravated Damage: Any Sphere charged with Prime 2 + point of Quintessence; Fire or Electrical (Forces) Effects; Vulgar Entropy 4+-, Life-, or Prime-based Effects that directly disrupt the target's Pattern.

Recommended Optional Rule: Dividing Successes
It is encouraged to allow players to divide up extra successes between targets, damage/healing/gathering quintessence, and duration either using the official Optional Rule found on p. 538. For my own ECC campaign this is replaced with a minor house rule: Rather than losing the "default" damage found in the Damage, Durations, and Targets chart and purchasing every level of damage on a 1:1 basis, players are able to receive the full damage of two damage per post base success they invest specifically into damage.

If players wish to add damage, duration and/or targets to their Effect before rolling, they should add up the total successes this would require and then include that in their Base.