While mages are powerful beings, they have one major drawback, past their mortal forms: Paradox. The bane of magicians, Paradox strikes all mages at some point in their careers. As the mage twists the universe to his whims, the universe bends back and strikes change on the mage in return.
Gaining it:
When casting Vulgar magic, a mage gets 1 point of Paradox per the highest Sphere rating used. When casting Vulgar magic in front of unenlightened witnesses, they gain an extra point.
Botching a Coincidental effect garners 1 point of Paradox per the highest Sphere rating used. Botching a Vulgar effect garners the same amount as normal, plus one. Botching a Vulgar effect with witnesses garners double the usual amount (2*Sphere + 2).
For Extended casting, each additional roll on a Vulgar effect garners an additional point of Paradox for botched effects.
Casting in a mage's OWN Sanctum (not another mage's) will make a vulgar spell coincidental, and thus will only accumulate Paradox on a botch.
Losing it:
A mage bleeds off 1 Paradox point per week since the last point gained, if it doesn't backlash right away. That is, if more Paradox is gained before losing any, the clock is reset.
Prime 5 can be used to ward against Paradox, as described in the Sphere.
Certain Wonders or Familiars have ways to deal with Paradox as well, as noted in their own descriptions.
Generally, as soon as a mage accumulates points of Paradox, they backlash on the mage. A Willpower point can be spent to stave this off until after the current scene, but this of course can be very dangerous. Sometimes, though, especially if the mage only has a few points (less than 5), the Paradox may linger instead of immediately backlashing.
A mage's player rolls their accumulated Paradox whenever it does backlash, and the results of this depend on the amount they've built up, and the number of “successes” they roll.
Paradox Points
Backlash Effect
5 or less
Roll Paradox and take the number of successes rolled as Bashing damage. The mage may also gain a trivial temporary Paradox flaw.
6 to 10
Roll Paradox and take successes as Bashing damage. In addition, the mage gains a minor Paradox flaw of some sort that probably applies a +1 to +3 difficulty to certain actions for a number of turns equal to the Paradox accumulated.
11 to 15
Roll Paradox - 10 as lethal damage. In addition, take a moderate Paradox flaw which might cause up to 4 points of difficulty penalties or just hit the mage with something bizarre otherwise.
16 to 20
Roll Paradox-10 as lethal damage. The mage probably suffers a Severe Paradox flaw or a similarly incapacitating side effect (Attracting Paradox Spirits, for instance).
21 or more
Take Paradox-20 aggravated damage (not dice, just straight damage). If the mage survives, he suffers a drastic flaw, attracts dangerous paradox spirits, gets shunted into a Paradox Realm, and/or finds himself in Quiet. He also will probably gain permanent Paradox, too.
Backlash occurs in several different ways, as listed below. This is generally decided by the amount of points accrued and the decision of the Storyteller (or Player if in a non-ST'd scene).
This is fairly straightforward. The only thing worth mentioning is that only bashing damage can be soaked, even if the mage has magical ways to soak lethal or aggravated damage otherwise; warding off a punishment for magic with more magic just doesn't work.
Paradox flaws are basically supernatural side effects related to the magic cast, that inhibit the mage in some manner that's appropriate. Paradox flaws will generally only affect the mage them-self, but others can notice it, and the most severe may hinder others as an indirect side effect.
Trivial Flaws: These are barely noticeable or slightly inconvenient, and probably lasts a short time. The mage's watch may run backwards due to Time effects, or his hair may stand on end due to Forces, for instance.
Minor Flaws: A minor flaw may be troublesome and may stick around for a while, but it won't usually be dangerous. The mage's feet may stick to the ground for a turn due to Matter tinkering, or a Life effect may cause a sneezing attack. These generally cause a difficulty penalty of 1 or so for certain actions.
Moderate Flaws: Moderate is a relative term, since these flaws can be dangerous and hamper the mage significantly and obviously. A Mind effect may cause the mage to suddenly only speak in a jumbled mess, for instance. These can cause a much larger difficulty penalty to a variety of tasks.
Severe Flaws: Severe flaws are, well, severe, and incredibly obvious or dangerous. The mage might cause all cloth items that come nearby to writhe uncontrollably, or in his own case, dangerously (Choking or binding the mage), for instance.
Drastic Flaws: These are the nastiest ways magic can backfire. A mage might suddenly start uncontrollably firing off magical attacks at himself or others, he may lose an Attribute or ability point due to some physical or mental degradation, or he may gain some minor Flaw as a permanent problem.
Paradox spirits embody the Consesus' desire to keep reality normal and uninterrupted. They tend to show up only when a mage suffers a significantly nasty backlash from paradox, and they rarely physically manifest. Generally, they'll work to correct the problem and hamper the mage that attracted them. Generally, the bigger the backlash, the more powerful the spirit.
No Spirit magic below level 5 (But only Level 3 for Summoning/Binding/Warding, though a Sorcerer rarely ever sees a Paradox spirit, given they don't suffer from it) can affect a Paradox spirit, other than to cause direct and vulgar damage (Compounding the problem). In addition, even a mage with Spirit 5 needs twice the successes as normal.
For the sake of Crossover, Garou have no trouble affecting a Paradox Spirit with their normal gifts and rites; as creatures of static reality, the Paradox spirits have no special protection against them. However, that doesn't mean they won't be angered if their tasks are interrupted. Most shifters see Paradox spirits as an odd type of Weaver spirit, punishing Namers for altering reality without permission. (Hidden tok-en 1O)
Paradox realms are places for the worst offenders of vulgar casting. Generally, a Paradox Realm reflects the sphere and specific effect used to cause the problem, and using magic to try and get out likely makes things worse. A mage must either wait out their punishment or find a mundane solution, depending on the specific case.