Magery and taint are directly opposing powers, which has some very important consequences for the people who have either. Very little of this information is actually known; however, it may have an impact upon a character's history, their relations, and the consequences of IC events.
Because magery is tied to the sun, the mage talent is more prevalent in equatorial latitudes, and rarest towards the poles. This means mages have always been somewhat common in Istria, and rare in Kalmar, which influences how each culture regards them.
Mages are, by their very nature, resistant to taint. If exposed to the comet's power by traveling into the Fringe — or worse still, the Broken Lands — a mage will lose power rather than gain taint. They will drop levels, retaining their knowledge but losing the ability to implement it, piece by piece. If the exposure is long enough, then the mage will lose all power and become normal — after which they are as susceptible to taint as the next person.
Although the talent for magery can manifest spontaneously in a human population, no mage may be born of tainted parents, even if all they have is latent, background taint. If one parent is a mage and the other tainted, then the heritage of the child depends upon their relative levels; if the mage's level outweighs the taint of their partner, then their children may be normal or mage. If the tainted level is greater, then the children will be tainted. If both parents are mages, then their children will be mages; if both parents are tainted, children may well be more tainted rather than simply intermediate in level, because the background taint increases with every generation of exposure.
Due to these two factors — the capacity to lose power and the influence of taint upon inheritance of talent — the population of mages has begun to dwindle over recent decades. Though the common populace isn't aware of the fact, the mages know quite well that each class is slightly smaller than the one before, and their average level of power is just a bit lower. They haven't figured out how to deal with the issue, however.
A person can become tainted in one of two ways: either born with it, or through exposure to a high level of the comet's energy. In Istria, the ambient level of energy is too low for spontaneous corruption. This is also true throughout most of the Fringe, but not everywhere; sometimes, people walk into a place they shouldn't and come out changed. It is more common by far for the taint to collect in a person over their lifetime without affecting them, and become more concentrated in their children.
