Magic, as taught in this verse, is Chaos. There is the Natural Order, and its opposite Chaos or Magic.
This can be viewed as a metaphor for violence or pollution (this is in some ways similar to the Defiler discussions we've had before). I was inspired by seeing the Gandhi quote on a lefties bumpersticker 'An eye for an eye makes the whole world blind', and by Lars Walker's book 'Blood and Judgement' in which he talks about an accumulating snowball of nastiness rolling down history, and hitting Christ hard enough to kill him, without him passing it on to others.
There are potential problems with each statement, for the first runs perilously close to the mess that is 'cycle of violence', but there's some use to these statements as well. Soooo....
In Charienteuse, workers of magic rarely cast spells, and when they do, they try to cast the weakest ones with the most minor principles behind them that they can get by with. On occasion, its better to cast from a Major Principle with a Minor Effect than a Minor Principle with a Major Effect.
Every casting of magic bleeds chaos into the world. This chaos takes the form of random mutations to chimera, of sudden and inexplicable violence, of plagues, and hailstorms. When it gets more severe, it starts to localize and pool, and uncanny areas form where the Rules of Nature are degraded and bent. Hideous pain-filled monsters that should not live a minute live centuries by strange pools in the deep wood, and call out with their minds for sacrifice (human of course.) Such creatures are frequently crosses of an ordinary animal and a snail, and they grow as decades pass, they grow in size and in wit and in cruelty.
Frequently, the only thing that can stop such a monster is more magic.
Occassionally, great acts of magic, or centuries of misuse, or the deliberate acts of Rogue Willworkers result in a total breakdown of Natural Order. This is a subuniverse where the reigning deity is anarch, and the magic high.
Such Holes are filled with pulsating colors that one can smell and taste as well as see, with ground that is like flesh, and air that stinks of acid or corruption by inconstant turns. Such Holes give birth to Chaotic Daemons.
Some Daemons are Instantiations. These are capable of causing great trouble, and their every breath is Chaos. Mortal blades, except when wielded by the most pure of heart, or royal blood are of no use. Magic is typically necessary to deal with them.
Not dealing with them allows murder and mayhem and forest fires and the enlargement of the Hole. But eventually, they return to tehie own world as Immaterial Concept Beings.
However, Incarnations have all the disadvantages of Instatiations, and one more. They are rooted in this world. Banishments that would rid the defenders of an outsider have no effect on them. They are immortal until killed. And they can have children.
This is a world with a tipping point. At some point, X+X=X is going to get so bad that the Willworkers will be slinging major spells every day, and thus creating their own problems later that week. The Natural Order is going to rapidly decay at that point, and the whole world will become Chaotic with no place for humans except for the strongest Willworkers.
One question is the recovery time from Chaos.
1. No recovery or millenia long. You're doomed. It may take a long time, but Doooooomed.
2. A few years might fix the scar....With proper conservation, and probable bueraucratic controls on the use of magic ('Fill out Form 10 X: Emergency Use of Magic...') you might survive quite well.
3. A few days....a pretty light hearted world suited for buckling swashed. Sure bad things happen if you're an idiot or desperate, but a reasonable willworker should not have too much problem.
Naturally, the verser can have his own magic which leads to an interesting point. magic can be a metaphor for a certain idea. Choose an idea with some good points, perhaps (or not), turn it into magic. Have everyone follow this idea. Have the hero come along and disprove the magic by using another kind based on another idea.
As usual, this is creative and interesting on your part, and on my part I'm concerned with how to make it work mechanically.
What strikes me as easiest in this case is to have this effect not be a part of the indigenous magic system at all. Instead, these chaos effects are the result of the direct action of the anarch ruling power, who personally violates the natural order in seemingly random ways in response to every use of magic in his universe. He allows the mortals to believe that these perversions are their fault because this serves his ineffable ends.
I like this explanation because it means I don't have to come up with the magic bias of the chaos drawback (which would affect the bias of all skills including it, namely: all indigenous magic skills), nor do I have to eyeball a positive sitmod for the "limit." I especially like the latter point because I'm not even sure this is a limit - what if the caster likes perverting the natural order and eventually bringing about Armageddon thereby?
Note also that this approach would mean that this chaos effect kicks in when the verser uses magic, too - perhaps a nice little bonus if your players are getting a bit too cocky about the local rules not applying to them.
Keep in mind that if you're Rogue, then pretty much everyone wants to kill you. A rogue is a threat to everyone.
Kinda reminds me of a combination of ... the way Mage: The Ascension does magic (gather enough Paradox and the universe opens up and swallows you) and something out of Captain Planet (mages are like walking nuclear reactors). Not a bad approach, just, like Scott said, a little weird to implement.
Yeah, well, in some places if you're a necromancer everyone wants to kill you. Should people get bonuses for doing necromancy? If I give a bonus for something, I want it to be unambiguously detrimental.
Self-destructive deserves a penalty (even though people are sometimes suicidal); own-universe destructive, depending on how gradual, gets more ambiguous.
Scott raises a very good point about the limitation not being a real limitation--it does not really limit the caster except in a Malthus Tragedy of the Commons way (if too many sheep graze the commons they all get sick, but I will still benefit by increasing the size of my herd even in such a circumstance).
This is more like one of those limitations on the thinking of the magi of a particular universe: we all know that magic works this way.
One thing you could do, Scott, is push the bias high and assert that the uncontrolled part of every spell is leakage of supernatural energy--that the magi open the tap and control part of the flow, but at the same time there is magical energy coming through that is not being caught by their pipe. Attach to this a general effects roll, with the center of the table being split between no effect and innocuous results, values spreading from there increasing the power and making it more and more related to the character himself. At a roll of 30, the character has effectively both succeeded and botched; around a 20 or 21 he did what he wanted but something bad happened to him or his interests. Going the other direction, the 3 impacts him directly, positively, and powerfully, while up around 12 or 13 he got some benefit incidentally.
In this case it would not affect the verser, because he doesn't have "leaky" magic (except if he learns it under this world's expectations). It would also be a side effect that would stay with the magic as it travels to other universes, because it is part of the expectations of the user, an essential element in how magic works.
I think it saves you from two other problems you didn't address: if the anarch ruling power is performing these acts, someone has to determine what that power is doing, what his numbers are, and how likely he is to succeed. So you're probably back to using the GE roll anyway, and maybe also rolling a skill check for the action taken. Also, if these side effects are part of the expectations of the users, they really should follow those users into other universes, at which point you have the problem of the anarch power acting under the bias rules of the other universe. If, though, you make it simply the consequences of free supernatural energy, you can follow botch rules for the limits in what happens (e.g., the amount of power released should be reasonably related to the amount of power sought, except on rare occasions).
--M. J. Young