The Plague Priests.
The Plague priests are a cult that has spread throughout the lands of Gorgamelle. Few cults are as universally despised and feared as the Plague priests because wherever they go, they spread disease, strife and hatred. Certainly they are not without resources, although the majority of their priests travel in small bands, harried by both external forces and their own unspeakable burdens. However, certain members of this sect have occasionally been revealed in high places in society. These may be cultists who have gained their position while keeping their affiliation secret, or be persons who have converted. In either case, they use their positions to foment unrest, chastise the righteous, and generally make life as uncomfortable for others as lies within their power. The Plague priests have been used by many philosophers as an argument for the existence of absolute evil. Unlike most of those who have commerce with evil or dangerous powers, the Plague priests do not seem to seek wealth or power, except as a means to their inscrutable ends. They do not value their own comfort or, often, even their lives. They seem to live for nothing except the impulse to do harm. Even other sects which are widely proscribed, such as the Kasserim, are not regarded as wholly evil (although perhaps wholly undesirable) - most people would acknowledge that they perform their heinous deeds to further comprehensible goals. Not so the Plague priests. Thus, they are a mystery to almost all folk, and even some among the wise regard them as a trial sent by the gods - perhaps the result of some divine curse.
However, the Plague priests are poorly studied (for obvious reasons) and are moreover extremely secretive (also for obvious reasons), so little is known of them and most of that is twined about with legend, rumour and outright fable. Their true name is the Gatherers of Penitance and their faith espouses the view that humanity has failed its gods most grievously - both in the commission of ungodly deeds and also in its refusal to accept the rebukes of the gods with humility. They point to the fall of all the empires of antiquity as signs of the displeasure of the gods and explain the cyclic rise and fall of human civilisations as due to the continual returning to previous godless behaviour. The current existence of disease, squalor and pain is a sure sign of the withdrawal of the gods, who otherwise could easily eradicate such things. Their canon holds that when all humanity acknowledges that it has sinned and asks for forgiveness, then the gods will return and make the earth a paradise. In the meantime, however, the gods have given the rule of the earth over to mighty powers they call the "Five Evils" - Plague, Manstrife, Famine, Earthstrife and Mother of Horrors. Idols of the Five Evils are often mistaken as the deities of the Plague priests, but these powers cannot be worshipped as such, since they are intrinsically antipathic to humanity. However, they can be propitiated, perhaps long enough to ensure the repentance of humanity and the return of the gods (an event referred to simply as "The Recoming"). Thus acts of mayhem committed in the name of a specific power serve two ends - they might persuade the power to stay its hand briefly, and at the same time, by visiting evil on people, they urge them towards repentance.
The origin of the Plague priests is unknown. Some scholars speculate that they arose in the chaos that followed the downfall of the Empire of the Sun. However Beltran the Irontongued states in his authoritative "Forbidden Cults" that the sect is much older than that, although it gained many converts in those years. He claims that the cult dates back to the all but forgotten Dark Millennia. The Plague priests have always been zealous in sending converts out to spread the faith, and since these prosetylizers work in independent, closed cells to prevent infiltration of the parent cult, they often lose touch. Today, offshoots of the cult exist in many realms - some barely recognisable, others closely linked to the parent cult.
The cultists themselves have certain features in common. They have in common a secret language known as "Speaking of the gods". Even in sects that have been long separated, this language is closely guarded and maintained, so that the cultists can communicate, or at worst, recognise each other. Members of the Plague priests are also distinguished by a certain fatalism, and in most cases, stoicism as well. A cultist in a position of power has been known to flee to avoid discovery and then work for years as a rag-clad mendicant, spreading sedition among beggars without apparent woe. Although they have no objection to the use of weapons, and indeed, have produced many doughty warriors, most powerful Plague priests are sorcerers, since they normally act in a covert fashion, which drawn blades can only compromise. When blood is shed, they prefer it be by their dupes. The cult gets its popular name from the potent sorceries they have developed to cause disease - a very efficient way of spreading misery. The actual message of contrition is rarely revealed as coming from a Plague priest - rather, members often masquerade as belonging to other religions, or as good samaritans, who in the wake of devastating wars or plagues, preach a creed of fatalism and repentance. By and large, such a message often attracts the approval of the powers that be. Other cultists join poor or dispossessed people and instruct them to rebel - to overthrow their masters and to share wealth equally. The inevitable wars and massacres create fertile ground for the preachers of repentance and further one other goal of the cult - the overthrow of established authority. Since most authority leads to stability and inevitably complacency in the population, it is viewed as antithetical to the movement's goals. They therefore work to make tyrannical regimes worse, and to destabilise more moral realms.