The Veil Treaty has often been compared to the United Nations by contemporary scholars, despite predating the UN's formation by eleven hundred years. The Veil Treaty was an agreement signed by representatives of various supernatural factions in 821 AD, swearing to limit their interference with the mundane world.
The Veil itself part ritual and part political agreement. As such, it didn't cover the whole world immediately. Instead, it flowed out from the Middle East along with the knowledge and culture that came along with the Islamic Golden Age. The Renaissance was sparked in part by the Veil Treaty finally reaching Europe. When new lands were discovered, ambassadors for the Veil Treaty would come and meet with the supernatural communities of the region and explain the way the world would now work--and the consequences for violating these new rules.
Today. The Veil Treaty is based out of London, after moving from Baghdad to Cairo to Fez. Originally, the Veil Treaty mostly concerned itself with ensuring that the Veil wasn't breached, enforcing the newly established separation between the magical and the mundane. However, the Veil soon became the venue for disputes within the supernatural community to be resolved, and then the representatives of the supernatural community to mundane governments. By the time the Veil Treaty relocated to London, it had become a sprawling bureaucracy with ties to governments from Beijing to Lisbon.
The modern the Veil Treaty is still concerned with maintaining the separation of the mundane and magical worlds, and it has agents still devoted to ensuring that the Veil is enforced. However the majority of its efforts are in providing services to the supernatural communities and handling issues on behalf of mundane governments. The Veil Treaty acts as the go-between for the powers of the mundane world and those of the supernatural world, keeping the delicate peace.
Modern members of the Veil Treaty include:
The Archangel Council
The Byfrost Group of Companies
The Children of the Blood
The City of Jewels
The City of London
The Committee of 300
Death, on behalf of the reapers of the Underworld
The Draculesti
The Druids of Avalon
Gaia, the spirit of the Earth
The Invisible College
The Kingdom Under the Mountain
The Knights of Solomon's Temple (the Templars)
The Lycan Nation, as represented by the Court of Wolves
The Magnus Foundation
The Mount Order Society
The Order of Assassins
The Orders of Atlantis
The Order of the Rosy Cross
The Parliament of Shadows
The Priory of Scion
The Seekers of Thoth
The Skull and Bones Society
The Society of the Green Dragon
The Society of the Illuminated Eye (the Illuminati)
The Summer Court of Faerie
The United Nations
The Vril Society
The Winter Court of Faerie
Though headquartered out of London, the Veil Treaty has become rather decentralized over its millennium of existence. Most issues are handled by a regional branch of the Treaty's operations, called a Veil Council. There are several hundred Veil Councils in the world, ranging in size from the great councils of Paris, Hong Kong, and New York to one guy who sits in a small office in Jamestown, St. Helena. Veil Councils are a place for members of the supernatural community to meet, solve disagreements, obtain service they are unable to from the mortal world, and the like. They're something like a shadow government for a region, regulating the world of magic just as a civil government regulates the mundane world.
And just like mundane governments, Veil Councils differ depending on where they're located. Most Veil Councils are urban affairs, given where most supernaturals live, and most cities with more than a million inhabitants has their own council dedicated to the city and its suburbs. There are also rural councils who hold sway over large geographic regions and represent the concerns of those supernaturals who still prefer the rural life. Even small towns have have a bustling Veil Council if they control a region with significant supernatural activity, such as Iqaluit in Canada, Timbuktu in Mali, and Petropavlovsk-Kamchatsky in Russia. Indeed, Ushuaia, Argentina--on Tierra del Fuego--has one of the larger Veil Councils in the world, given how its territory includes all of Antarctica, and all the various mysteries buried under the ice.
The Ushuaia Veil Council is something of an exception, however. Most Veil Councils keep their territory strictly within the borders of one mundane nation--to do otherwise creates legal and jurisdictional uncertainty, especially for mundane governments who might not trust an organization located in another nation. Thus, every nation has at least one Veil Council located in their capitals--even small island states have at least a token Veil presence.
The Veil works as something of a minor psychic suggestion, encouraging humans to ignore the supernatural, or to forget anything odd they've seen, leaving them with only odd memories and an unsettling feeling. Over the years, countless mages and artificers have learnt how to tap into the Veil's magic to hide their stores, towers, or sanctuaries from the eyes of humankind. Entire buildings have been veiled away through cleaver applications of thaumaturgy. Likewise, numerous supernatural races, such as dragons, fey, jinn, and horrors have learnt how to use the Veil to create human forms for themselves as a form of limited shapeshiftng to better disguise themselves within human society.
Of course, the Veil isn't perfect. Humans pierce it all time, naturally falling through gaps in the weave and seeing the truth for themselves. Likewise, the more a human is exposed to magic, the harder it is for the Veil to grab them and encourage them to forget again. A mage flinging a fireball in a busy mall, or a werewolf shapeshifting in a crowded street is likely to cause what is known as a Veil Breach--when numerous humans break the Veil's hold on them, and suddenly awaken to the truth of the world.
That is where the Veil Treaty's agents come in. Though formally known as the Blackcloaks, they are more commonly known as the Men in Black. They investigate paranormal happenings across the world, and seek to restore a sense of normality, patching the Veil where they can, encouraging people to forget, modifying memories so the Veil can once again grab a hold of the awoken human.