So, as I'm currently hip-deep in editing A Murder of Mages, one aspect I'm weaving in there is further explanation of Mage Circles, which is a crucial part of how magic works in Druth culture. Well, not in how magic itself works, but how the culture deals with magic.
Druthal is a civilized and enlightened culture that's always had a troubling relationship with magic. Up until the beginning of 11th Century*, most of the people were openly hostile to mages. A tolerant attitude about a mage at the time would have been, "Let's only throw just enough rocks at him to chase him out of town."
But over time, society advances: laws are codified, law enforcement is formalized, and justice is no longer enacted at one man's whim. With that, attitudes about magic are dragged by the scruff of the neck, kicking and screaming the whole way.
So you have a society where some people loathe and fear magic, but others would happily employ the services of a professional mage, as much they would a doctor or lawyer. Just as doctors or lawyers have professional organizations and formalized accreditation, mages need the same, and in Druthal that takes the form of Mage Circles.
A Circle, in simplest terms, is an organization of mages that certifies that its members are trained, and assumes liability for its members actions. They could be a tightly knit handful of mages, with similar goals and intentions, bound together like family. Or it could be a sprawling guild of hundreds, with only loose interaction, functioning mostly as an institute of insurance.
The main function of a Circle is, in theory, to ensure that a mage amongst their ranks knows how to handle their power, and that there is a system in place to bring them to heel should they get out of control. In actual practice, a Circle often serves as a buffer between an individual mage and a potentially spurious legal system. They will protect their own-- fiercely-- from police action if they feel its unwarranted, and often still provide counsel and support when it is justified.
What this also means that most mages, when encountering an Uncircled mage, will react the same as a doctor would in meeting a back-alley surgeon whose only training was watching reruns of Marcus Welby.
However, since there are scores of Circles in the city of Maradaine alone (most with less than ten members), mages in general are hardly a unified political or legal body. There are plenty of professional rivalries, simmering enmities and simple open hostilities. Most of them don't get along with each other, and they certainly don't get along with law enforcement or other government agencies.
Which means when a mage is killed in a horrific, ritualistic murder, and one of the Inspectors investigated it is also an Uncircled mage, then you're going to get... well, you'll get A Murder of Mages.
*- Thorn, Murder of Mages and other Druthal-based projects I've been working on are set in the 13th Century, specifically in 1215.