There is a very famous saying.
If you gaze at abyss long enough, it gazes back at you. To be honest there is some truth in it.
This world is divided into four layers. First is The Seen. The ordinary world. World of humans, animals or in other words something that can be 'seen' or 'perceived'. The residents of this world are mostly ordinary. Living ordinary, simple life. But there are exceptions. A few.
Then there is the second layer, The Unseen. The world of creatures unknown. Those who have the ability to transform, shift and create things from nothing. Vampires, Fae, Elves and beings known as myths or legends in the ordinary world.
The third layer is The Divine. Gods, Angles and Heroes. The world of divine, order and supreme beings.
The fourth layer is The Abyss. The undead, ghosts or lost souls reside here. The world of the Dead. It is considered the dangerous of all the realm.
Residents of The Unseen can enter the The Seen world and influence in some way. The Seen can only percieve the other layers only if they get in contact with them. The Divine remain where they are but have influence on the The Seen and The Unseen. The Abyss isolated from the other three, is perceived as the most dangerous of realms. Considered a threat even by The Divine.
In this world where many things are unknown and hidden, if one encounters something 'mysterious', there is a chance they have fallen into the hands of abyss. Only those who tried to gaze at the secrets of this world and far beyond, intentionally or accidentally, whether dead or alive, mortal or immortal, have the chance to 'know' the 'unknown'. And only the brave among them come back while the rest vanish without any traces. And to mantain the balance between each realm another force is deployed by he consciousness of the The World, known as The Keeper. The one who keeps the secrets of the world. There identity is a secret even to the most supreme of beings.
If you gaze at abyss long enough the abyss gazes back at you. Legend says that if one encounters something mysterious, and if they are lucky enough, it might be one and only chance for them.