Factions
Voidcults are mostly small, seldom organized (due to the madness of their members) and often rather short-lived. The Black Moon is different. 
The members are mad, like everyone trying to serve entities from the Void. But they seem to be a different kind of mad, if such a thing makes sense at all. Their members seem to be normal members of their community - until they commit crimes so horrible, any sane mind simply cannot comprehend them.
Nobody knows how long the cult already exists. First accounts seem to date back to the first days of the Two Empires, but maybe it already existed in the Empire. That is also rather unique, as the madness of these cults normally lead to their demise.
Organization
The cult is organized in small chapters, consisting of about 10 to 20 members. In the old days, those chapters were mostly found in cities, where they could meet in secret and keep their normal facade.
Today, many chapters have taken over rural communities, often by slow corruption of the inhabitants. Those strong enough to resist are sacricifed.
Every chapter is lead by a high-priest. Disturbingly, many of those are Theurgs, sometimes even still in service. Their magical knowledge makes them very valuable to their masters from Beyond, and often they already have been corrupted by Magick itself.
Members
Members come from all walks of life, but those in charge almost always possess magical power. Theurgs seem to be especially susceptible to the calls from the Void, and behind their robes of office, they are able to hide very dark secrets, it seems.
There are rumours about dragons that form a kind of concilium within the cult, but these stories are denied by most scholars - and dragons themselves. No dragon would ever stoop so low as to side with the Void. Or would they...?
After the First Emperor was killed, the scholars of Magick fell into disorder. With their guide and ruler dead, they had to find a new place within a new society. Soon, the transfiguration of the First Emperor began. He was now not only the founder and ruler of the Empire, the most powerful archmage to ever arise from humankind, he became something more. 
At first, he was revered like an important ancestor to all the wizards, but slowly, a cult arose. Some scholars declared, that his death was not final, that someone as powerful could best even death itself.
And then the visions began. At first, they were thought of dreams and hopes (sometimes even madness), but more and more wizards seemed to be touched by an intelligence. They heard a voice talking to them, ordering them to form a new order of Magick, to stem the corruption and teach new mages to cope with the dangers.
About 300 years after the Fall, a new order arose in the Southern Empire, calling itself the Theurgy. They declared, that the First Emperor had not died, but that he had been transformed into something higher - a god. The only god.
Their magical power seemed to prove their teachings, as they seemingly could control the corruption of Magick. They set out to help the people, make their lives better and heal their ailments.
When the Empires finally clashed, they had already established temples in the north, and got a firm foothold there. The Plague even made their influence stronger, as the populace turned to them for help. The Theurgs were sure to find scapegoats - Wild Mages, they call them. All those that do magic without being a Theurg.
And so, the witch hunters were born, and the pyres burned high, not only with the bodies of the dead.
Organization
In the beginning, there was one High Theurg in Tarratoria, and every Consul (leader of a major temple) would do his bidding. But today, there are High Theurgs in Tarratoria, Edesus and Naxeia. In Olbomenus, Sophanis II calls himself Master Theurg. There are no more Consuls, as everybody seems content to rule over a small area instead of being ruled by someone else.
Small temples are run by a Priorus or Subpriorus. These Theurgs hold considerable magical power, but often do not have the means to get their hands on spellbooks of greater might.
The smallest shrines in villages are run by a so-called Canonicus. Some of those low-level Theurgs even travel the roads, trying to help the people and maybe accumulate enough magical knowledge to rise in the ranks.
Founded about two hundred years after the Fall, the Order of the Wyvern is a haven for Imperial Knights that found themselves without an Emperor, without an Empire and without any perspective to their life.
The Order tried to revive the spirit, if not the cause, of the old order, to protect the people against the monsters from outside and within the borders of the former Empire.
With the crimbling of the successor-realms, the order also fell on hard times. Today, only a few stalwart nobles hold on to the wyvern-crested shield and banner.