Corruption

That which breaks the rules of reality, is referred to as Corruption. Breaking these rules is both powerful and dangerous. 
Corruption creates instability. This instability can be useful, it may allow a complex spell to be cast with no effort, or grant a supernatural power to someone who comes in contact with it. But corruption always has unknowable side effects, the complex spell may create an anomalous scent of blue berries, and that supernatural power may give its user cancer a year after attaining it. This instability is often compared to entropy, but on a much larger scale where local reality itself is breaking down from order into disorder.
The instability from corruption propagates from its source causing more and more anomalies, this process is often slow, depending on the source of the corruption, with small effects such as the casting of a single corrupted spell creating very little permanent damage. It is only when very corrupt entities mar the land or many corrupt spells are cast in a single location that a seed of corruption can be planted and begin to grow.