Erithians

'Erithians' is a common name applied to inhabitants of the Hidden World of magic. The name itself seems to be truly ancient, and according to many its source is a name of a kingdom where magic was first discovered. Which happened sometimes around the neolithic revolution, somewhere in the Middle East.
Over time the Erithia ceased to exist and disappeared from history (if it ever existed), but it began living as a self-perpetuated myth, a common denominator of all magic users. Magicians are citizens of Erithia, even if the country no longer exists and nobody even knows where it existed (or if it existed).
Of course, even that mostly ceased to be, as in time Erithia changed its meaning. Being closer to an adjective than a noun. There is no longer any sort of unity among Erithians - despite claims of the remaining adherents of The Ascending Path, unity of Erithians was long death even before the Christianity came to be.
The Hidden World is no longer hidden, though. It made a devastating comeback during the Second Battle of the Marne. Now, the erithians are fusing with the countries/alliances of the mundane world. And only time will tell how they will affect it.

Structure

Majority of today's Erithians hail from the communities that signed the Treaty of Vatican. This means that they fully accepted the fact that they should live separately (to a point) from the real world... and do not attempt to use their supernatural powers to influence it - while banning Black Magic as a whole, as well.
Treaty of Vatican set the proper 'organization structure' of the Erithians. Separate communities hidden in the various places in the world. It might be a hidden 'club' for sorcerers hidden behind a special door in some downtown club. It might be a whole village built in pocket dimension, that can be accessed only by certain road (and only by people with a magical spark, or those that accompany them). It might even be whole little country (only few of them worldwide) with several different entries, practically written 'atop' a region.
That's where the Erithians retreated to. Those that adhered to Treaty of Vatican, that is. Magical flora and fauna - in civilized areas - were pushed into dedicate 'natural reserves', a hidden pocket dimensions of varied size as well. There might be still supernatural creatures living here and there, in depths of Africa or in an Amazon rainforest, but all beings of magic were either send into pocket dimensions or - if they resisted that - purged.
The Treaty forbad making attempts to organize international societies of the erithianss. This was done to prevent any of them to raise too powerful. Separated, they overtime grew to be a slightly different citizens of their respective countries. Patriotism and nationalism being the best defense from them once against growing into worldwide control. Trying to have Erithians from France and Germany cooperate with each other was about as possible as making their countries ally. Which means not being possible at all.
On the other hand, it also meant that there was no pan-European organization to try to prevent the local groups of Erithians from falling into the control of the Forbidden - several of them, while officially adhering to rules of the Treaty of Vatican, were actually actively seeking to subverse it, and were responsible for the end of Masquerade and the Second Battle of the Marne.

History

Erithians 'began' before the written history did, and because of that, even they do not truly know their past. They were already around when Sumer began, and seem to mostly originate from Middle East region, especially Mesopotamia and Egipt (somewhere there Erithia lied). They were high priests there, using their power to keep themselves in power. Few hundreds of them at best.
They weren't in any way evil. Their position was the natural effect of their powers. They ruled the humanity, but didn't precisely guide it. There was no central thought to their actions. Many of them genuinely believed in the causes of 'mundane' people surrounding them. Cities they had influence over fought each other quite often. When times changed, they changed as well.
They were behind the thrones of Assyria (whose bloody way were partially caused by some black magicians considered evil even by their own kin, and in the end taken down), Sumer, Babylon, Egypt and Persia. They cared little about the lands outside of their domain... until the Greece came and took over their Empire.
They persevered... and adapted. It wasn't long before they were influencial people of the diadochi states. Focused on themselves, they once again failed to notice a potential outside threat... though it wasn't as bad as it used to be.
Roman Republic quickly draw their attention. A 'mundane' state, that somehow defeated Carthage, ruled from the shadows by the exiled 'extremist' black magicians, responsible especially for the human sacrifices before the throne of Baal. It was then that the erithians infiltration started for real, though some of that began earlier, just not in organized way - erithians were naturally interested in powerful countries.
It was the start of the so called Second Mage War. Nobody knows when the first one was, the knowledge about it faded in history later on.
Rome resisted the onslaught of eastern cults for quite a while, great example being the condemnation of Bacchus cult in 186 and execution of 7000 of their participants, many of them being erithians. Roman Republic itself - nor the Roman Empire - truly knew what it was facing, they didn't even know that there was war. There were hidden groups, a bit later united into the Unseen Legion.
It resisted wave of eastern influences - together, unknowingly, with magician influences - for a long while. Sometimes it happened through laws, and sometimes through wars - though eastern magicians soon understood that they can't defeat Rome in open war of mundane arms, and all they could hope for on that field was holding their 'motherland' of Persia away from its political control.
Some magicians abandoned their old motherland, becoming Roman in true sense of the world. Helping - from shadows - in facing the threat of falling under Erithians total control.
Second Mage War ended with ultimate, but brief, victory of the Erithians. Severus dynasty was an epitome of eastern influences. After it fall, the Rome - increasingly eastern in its political system - was under (more or less...) control of the Erithians. Even the coming of the Christianity wasn't a problem at first, even if it was seen as a minor problem... until the Third Mage War started.
It grew stronger and stronger. Erithians repeatedly attempted to sabotage it - by both persecutions and attempts at promoting schisms. But both arianism and gnosticisms, best weapons of the Outsiders in that war, failed... the Third Mage War ended with surprising victory of ascendant Christianity. The dissolution of Western Roman Empire was a key point of that war.
When it broke, so did the unity of Erithians. New countries rose, all of them different and opposing each other. While first or second local generation could still be truly Roman, overtime the local 'nationality' and the point of view of the courts of local kings took precedence. No group can remain cohesive when the world changes as much.
Church - now vaguely aware of Erithians existence - had something they didn't. The know-how on how to run country. Arianism - to which most of the Germannic invaders adhered to - was slowly but surely eradicated. The attempts to infiltrate the Church failed - as though it looked like it succeded for a while, the Irelands monks and then a Carolingian Renaisance sang the song of resurgence and revitalization. Even using the eastern patriarchies as a staging grounds for attacks failed and misfired spectacularly.
What's more, this war ended with Erithians losing control over East as well, as while Eastern Roman Empire persevered for a while, the sudden assault from 'barbarian' Arabian tribes were something even the Erithians couldn't foresee. Persia fell, and the ERE changed into the shadow of its former self. While it took a while for the Erithians to infiltrate the Caliphate, it happened at the cost of losing their influences over what was going to one day change into Eastern Orthodoxy.
The Third Mage War ended together with Islam conquests. But it wasn't long before the Fourth Mage War started.
It was the longest and most bloody of the Mage Wars (considered by historians to be more of a time periods of Erithians history that true wars). It started during the saeculum obscurum, the period of the great fall of Papacy... that was, in fact, an effect of Erithians efforts at restoring their control of the West.
Harsh times beget strong and ruthless people. After the Rome fell, many of the western Erithians fell into Black Magic and ruthlessness, often under strong leaders that united them over the promises of restoring their great past. Truly the lowest they ever fell.
What was expected to be a swift victory, changed into a disaster. Saeculum obscurum changed when monks of Cluny led the movement for reforms and the influences of the Erithians were all but annihilated. The same happened in the East, after Erithian-sponsored Iconoclams fell. When it seemed that the West will be outpaced by much stronger Islam an the Byzantine Empire was about to fall, the West suddenly rose in a succesful counteroffensive. Crusades reinvigorated the almost collapsed Byzantine Empire, and the Crusader States served as a pipeline through which the achievements of East were brought to relatively outdated West, strengthening it immensely.
Islam was pushed back from Italy, France and - finally - Spain. Byzantine Empire persevered for several centuries. The greatest attempt to bring dissolution to the West fell when Catharism was destroyed by another crusade. Through all this time, the secret war was waged, and society after society, the mages were defeated and eradicated - or reformed. Black Magic was destroyed, while those that never fall to its vices were accepted and reformed, changed into weapon of the Church in this war.
Inquisition was created, making sure that further attempts of destabilizing the Church by spreading internal dissent. Soon it was also brought into the secret part of war, secretly hunting the still fighting groups of black magicians.
The Fourth Mage War was all but won when the Reformation happened. Ironically, the power of the Anathema (as the black magic users began being called among the reformed Erithians) waned to the point that they had no finger in its creation. But it saved them, at least for a while and only in protestant-held parts of the Europe. And at least until the new churches discovered them ... and didn't like them one bit, leading to violent witch hunts. That, unfortunately, hit both evil and 'reformed' Erithians, but mostly eradicted the former ones.
The official end of Fourth Mage War happened when Treaty of Vatican was signed, which happened in 1560, around the time the Council of Trent was held and the counterreformation movement started. Church won. Communities of Erithians in Catholic dominant lands were pacified and supported the Church in its war for quite a while. Those in protestant lands were practically eradicated, with those in Islam world being the only potential threat - but one that couldn't be won without defeating Islam as a whole. What's more, those Erithians that ruled them never suffered from the slippery slope effect that brought the West mages into black magic after the Western Roman Empire fell. They were a danger to body, but not soul.
The division happened, and the Church was supposed to have guarded the newly established communities from the so called Forbidden to return. It failed, though it's hard to blame it. The Forbidden were mostly forgotten and this opened new possibilities the few of them that survived. The tide of secularism and the French Revolution dealt a decisive blow to the 'secret' part of the Church and its control. Protestant attempts of playing the same field were mostly stopped during the Enlightment, when many of the 'enlightened despots' dismantled most of the power their churches had, transfering it to the various states of the period - the same happened to Russia.
This allowed the Forbidden a much easier infiltration, which in turn led to the Second Battle of the Marne - and the start of what many Outsider historians see as Fifth Mage War. It is unknown how much connected are they to the moden 'ezoterism' and various occultism movements, but at least some of them seem to be connected to the Forbidden as well.
With Church in deep crisis, their control even over the Treaty of Vatican adherents ultimately faded - many of them, however (especially those in Catholic states) began looking back at it since the masquerade was broken and the true extent of Forbidden power was showed. What's more, the supernaturally militant branch of the Church mostly collapsed as well, though signs of recent resurgence are visible as well.