The Veil & The Waning of Magic
In my setting, there are several layers to "reality" -- the main being that which all un-initiated humans belong to (The Mundane), the world which supernatural beings and humans touched by the supernatural exist in (The Magical or The Spiritual World), the thing that separates them (The Veil) and then several other layers which are less integrated with reality than The Magical. These other "layers" of reality include: Faerie, the Realm of the Dead, the Creeping Dark, The Infernal Abyss (Hell) and many, many others which are too numerous to name or describe here at this moment.
The "Veil" which separates the Mundane from the Magical is a somewhat more recent occurence -- in the Time before Time and up until the late middle ages, the world of the Mundane and Magical had no separation. They were one and the same and no special ability was needed to percieve the existence of things like vampires, demons or other supernatural beings. However, with the advent of Enlightenment beginning in the Rennaiscance and the "nailing down" of the boundaries of what is real and what is magical, the influence of the Magical on the Mundane world began to wane.
While many older Supernatural creatures refer to the Rennaiscance as "When the Veil Fell" this is something of a misnomer -- the veil falling and the Magical withdrawing from the world of the Mundane was a very gradual process, happening over centuries and only really fully coming into fruition in the late 19th century. And even now, there are places where the Magical and the Mundane are intertwined and difficult to separate -- places where the Veil is thinned.
This includes liminal spaces such as bridges, the hours of dawn and dusk, truck stops and etc., but it also includes places where Belief in the Magical is still strong and places deep in the wilderness where humans rarely tread where the reach of human Belief has yet to change the Nature of the world.
A notable exception to this gradual withdrawal is the Land beyond the mists, also known as Faerie. The Mists are separate from the veil though they are often mistaken for one another and in human Belief, may be the same thing. However, Faerie separated from the Mundane long, long before the Middle Ages, though the Mists certainly thickened as the Belief in Magic waned.
Faerie was separated from the Mundane world around the same time that Myriad Eyes and its ilk came into the World -- sensing the disaster that would come with this being, the Fae worked together to pull their strongholds into a separate layer of reality that was cut off from the World, hopefully safe from the creeping inlfuence of the Outside Things.
The exact function of the veil is a mystery to most of the beings which live in the World, though it seems to have a protective effect on both the Magical and Mundane worlds -- protecting the Supernatural from being noticed and hunted en masse by humanity which no longer truly Believes in them but continues to fear the Magical on a seemingly instinctual level.
It's important to note that the Magical and Mundane worlds are not wholly separated, unlike the World and the Land Beyond the Mists -- the Magical simply exists on a different layer from the Mundane. Both affect one another and anything that exists in the Magical exists in the Mundane and vice versa. It is the Veil which causes things that exist in the Magical to appear differently in the Mundane. A vampire's glowing eyes and fangs will not be visible to a fully Mundane being, a thriving bar owned by a faerie exile might appear to a Mundane mortal as long shuttered and abandoned.
There are some things that will always break through the Veil and things that are not protected from Mundane eyes by it.
Agents of the Myriad Eyes and indeed, the Myriad Eyes itself are not separated from the Mundane in the same way as other supernatural beings, likely because the Myriad Eyes and its ilk are not a part of The World, they come from outside of its boundaries and as such, are not protected by its "natural" functions. A werewolf who transforms into their hybrid form will be seen by a Mundane mortal, though the marks of their lycanthropy will be invisible in their human and wolf forms. A vampire who turns into mist or who is in the process of feeding will become obvious to a Mundane mortal who sees the transformation or who is fed upon (though the veil will quickly reassert itself after feeding). A witch who summons a gout of pale blue fire in the middle of a crowd will be seen by the Mundane beings within the crowd. Psychic imprints and echos -- hauntings -- exist completely independent of the Veil as well... And, of course, the Magical is easily seen in places or times when the Veil is thin.
On the rare ocassion that a Deity presents itself in the world in its True Form it will be visible to any Mundane unlucky enough to be present, as well.
Very few humans are wholly effected by the Veil -- most will be able to see ocassional hints of what exists beyond it, though they'll frequently dismiss or rationalize what they've seen. Those who Believe very strongly in the Magical, no matter where they live will also ocassionally get peaks beyond the Veil and can interact with the Magical world to an extent which other Mundanes cannot (this includes Spiritualists who genuinely believed in the Spirit World, practicing witches, some pagans and other strongly superstitious people).
Tl;dr: Human Belief has a strong effect on the World/Reality itself and the waning of human Belief in magic and the supernatural caused those things to start to exist on a separate layer of reality that is invisible in most circumstances to Mundane mortals.
The barrier between these two layers of reality is referred to as the Veil which can be thinner or thicker in various places or in some, does not exist at all.
The Nature of the world is that the Magical and Mundane are meant to be fully intertwined an in places where human Disbelief hasn't taken root, the World still exists as it did before the Veil began to fall.
The Magical is protected in most circumstances from the influence of the Mundane by this Veil and its purpose is to protect the Magical world from being destroyed by Disbelief and thus unsettling the balance of existence.
Magical beings are capable of manipulating the effects of the Veil to hide places and things (and themselves) from Mundane mortals or can step through the Veil should they desire.
There are certain circumstances which are too extreme or in which the Magical is interacting too directly with the Mundane world for the Veil to hide it -- such as a zooanthrope transforming into a hybrid/mixed form, a witch using flashy magic which cannot be rationalized in front of a crowd, a vampire's fangs immediately after feeding and etc.