The Dead Lands of Athas
Because magic is so powerful, every ruler controls it tightly. For the most part, it is impossible to maintain control of a populace dotted with numerous psionic individuals if one is not also a wizard. Thus, nearly every ruler on Athas is a wizard of some type, usually a Defiler.
Sorcerer-kings send their agents to destroy potential rival wizards hiding within their cities. Nomad witch-lords banish rival mages to the unforgiving sands of the desert. Halfling chiefs exterminate followers who show any sign of control over the supernatural. Even otherwise timid hermits have been known to risk their lives in an effort to make sure that no wizard enters their territory.
Despite their efforts, leaders seldom monopolize magic. There are always at least a few individuals in any society who practice it secretly. Of course, these powerful people are almost always the hand-picked, loyal followers of the leader.
In most cities (and many villages, tribes, and clans), there are secret leagues of Preservers called the Veiled Alliance. The Veiled Alliances are confederations of Preservers working together to protect their members from assassination and harassment by sorcerer-kings and other lieges. The members work together to shield each others identities from the authorities or to help those who have been discovered to escape persecution. They are often involved in plots to overthrow their oppressive overlords.
There are perceptively only two drawbacks to belonging to a Veiled Alliance. First, membership is permanent. When you join one of these secret organizations, you pledge to uphold its charter until death. Anyone failing in this pledge is cast out, and the alliance assigns one of its members (usually someone experienced in such matters) to assassinate the outcast. This seems rather severe to those in the ranks of the uninitiated, but it is a condition of affiliation that all members agree to when they join the alliance.
Second, all Veiled Alliances require that their members be Preservers and not Defilers. The reason for this is practical, not idealistic: even a few mages will decimate a small area if they do not practice their art responsibly. Any violation of this principle always results in the banishment (and subsequent execution) of the Defiler.
For the most part, although each league goes by the same name, they are in fact separate organizations, loosely linked by common goals. Most follow the two principles outlined above, and will extend their protection to a member of an alliance from another city (providing adequate proof of affiliation is provided). At the same time, they must be on constant watch for spies, for most rulers will stop at nothing to uncover and destroy an alliance operating within their territory. For this reason, punishments are sure and swift in any Veiled Alliance. It is better to err on the side of security than to risk exposing the entire society. They function like terrorist cells, whereby any individual knows a contact or two, and occasionally has veiled communications with other members.