Censor's Veil

Censor's Veil is the name the Initiates coined for the autonomous and homeostatic Masquerade that maintains the vestiges of the 'normal' world by hiding the supernatural from the majority of humans. No one knows who established it or when, but it's generally believed to be at least five thousand years old, as the oldest magical treatises mentioning it came from the ancient Sumer. In other words - they are as old as writings as a concept. Censor's Veil (or simply Veil) is at least as old.
Veil is less a concept or a spell, and more a natural law of the Reality itself, merely one that is kept hidden from mundane scientists by its very existence. It is also a double-edged sword. While it allows the majority of humans to live their lives in relative happiness and comfort, it also forces the few people excluded from it (namely, the Initiates) to face all the horrors of the universe on their own.
Many of the Initiates dream of the unveiled world. They believe it to be the way of giving Mankind as a whole a fighting chance by multiplying its available firepower. Others believe it to be the best way of making Mankind as a whole get noticed by the Powers - and thus a recipe for the end of humans as a species.

Manifestation

Censor's Veil is the manifested division between supernatural and natural. It is maintained through the acts of the Censor - a non-sentient force of nature capable of rewriting memories, and both present and past state of Reality to maintain the Veil and the relative cohesiveness of Reality as seen by non-Initiates. This might sound confusing, so it's best to describe it through practical examples.
Let's imagine that your neighbour has encountered a supernatural creature that has promptly killed him in some grizzly and unnatural way. The very fact that it happened is a threat to the Veil. Autopsy of such a body will clue the local police and whoever discovers the corpse that something impossible is happening in their town. Censor will then promptly intervene. The body itself will begin to look completely different to Initiates (people excluded from the Veil) and non-Initiates.
The former will see the truth, while the latter will see a body of a man who died of natural causes. And not just see - they can touch and analyse parts of the body that are no longer there. An Initiate observing such an event will see the non-Initiate freeze (or, perhaps, move their hands and tools through the empty air). The non-Initiate, in the meantime, will have cohesive and detailed memories of the entire process - which didn't happen. This is merely the lowest level of the Censor's Sanctions. It gets worse.
Sometimes it will erase the victim from the world of Non-Initiates. The horrible and traumatizing death of your uncle Billy (who accidentally touched a cursed object and was slowly eaten from the inside by a curse) will not open your eyes to things that you shouldn't know about if you no longer remember that uncle Billy existed. If every picture of him in your house (and elsewhere too) vanishes or changes into pictures of your other relatives. If it's no longer Uncle Billy who took you to that summer camp ten years ago, but instead it was your other Uncle, Sam (who, naturally, now remembers bringing you to said summer camp).
When things get even worse, so do the things that Censor will do to maintain consensus Reality. If a teenager leaves the group during the school trip and vanishes mysteriously, Censor will conjure his Shade - a material illusion of a person that was. Initiates perceive them as people rendered in black and white, while for the non-Initiates they will be indistinguishable from their original selves. Shades will continue existing until they can safely 'exit the stage' without damaging the Veil.
Said teenager might commit suicide (if it's cohesive with his past behaviour) a week after returning from the trip. Or he might continue living for three decades, before dying of natural causes. There is always some deeper, underlying logic to such events, although one that only the Censor itself is capable of comprehending. Discovering that one or more of your friends or family members are Shades is often among the most traumatic results of being Initiated.
If something disastrous happens to an entire community, and Censor fails to conjure an immediate (and mundane) explanation for it, the end result might be a village or a town populated almost entirely by Shades. Eventually, they will move out (and then vanish or die elsewhere), freeing the place for living people to move in - and thus the village will be repopulated. Or, perhaps, it will be destroyed in a Censor-born forest fire.
The worst Sanction is reserved for situations when nothing else can be done to maintain the Veil. When it happens, the Censor might erase entire regions. People who were born there will have their pasts rewritten. People who were still there when it happened will now be Initiated - at the cost of being practically imprisoned in the sanctioned area, for their very existence is a threat to the Veil. Maps will be modified, the entire region struck from them. The roads that led to the sanctioned town will now lead to the neighbouring town, with even the Initiates having to use magical means of accessing vanished territory.

Localization

There are no territorial limits to the Veil, as the Censor is fully capable of warping Reality at the very least within the confines of the Solar System. There are exceptions to it - namely the Initiates, people who can see the truth behind it... at the expense of being much easier to get spotted by things normally hidden by the Veil. What's more, certain glyphs can influence the Veil as well - it's common for Initiates to carry weapons at them at all times (at work, at school etc.), simply ones marked with concealment glyphs which makes them invisible to non-Initiates. Many of them are also similarly concealing their studies or saferooms.
Another notable exception happens when a Non-Initiate is attacked by something supernatural. If it's impossible to 'explain' the attack through mundane means (for example by displaying that monster as a human or animal attacker), Veil will be temporarily lifted for the victim. If they manage to resist the attack, the Veil will return, and their memories of the attack will be rewritten to maintain normalcy. If they fail... well, the mechanism for that was already explained above.