"Do not unveil the supernatural to the eyes of man."
This is the central tenet of the Veil Treaty, the document that ended Earth's magical age and gave rise to the modern world as we know it.
The Veil is an incredibly complex ritual that tapped into the lingering power of the Tower of Babel, as well as the many ley lines that ran beneath Mesopotamia, to rework the nature of magic itself. It encourages the vast majority of humanity to ignore the supernatural, to rationalize away the weird, the forget that which cannot be explained. The vast majority of humanity lives their lives completely unaware of the world of magic, myth and monsters that hides in the shadows. We pass off the stories of Atlantis and El Dorado as mere legends. Elves and werewolves are but the things of fairy tales. Ghosts and bogeymen are but the result of overactive imaginations. Magic is just primitive mankind trying to control forces they could not understand.
Those are all lies we tell ourselves, to convince us that we're not crazy, that we didn't see those strange lights in the sky late at night, that we never saw those inhuman shapes moving through the woods, that there really isn't a monster under our bed.
And the Veil helps. It encourages us to forget the unexplainable, rationalize away the strange, and make the magical mundane. It's a suggestion, whispering in all of our ears, encouraging us to pay no attention to that strange shop on the corner, to pass those ancient ruins on by, to see the man chanting to himself as weird and someone to be avoided. Only when confronted with facts we cannot ignore does the Veil come down, and we can finally see the Earth for the magical world it truly is.
But the Veil is far more than just a simple perception filter. Its magics can be drawn upon by non-human supernatural races to veil themselves in a human guise, it can be harnessed to cloak supernatural locations, it can warp space to create bubbles of reality only supernatural creatures can enter, it twists the mundane recollection of history to erase supernatural events, it prevents mundane humans from recognizing the ruins of ancient magical civilizations, and more. Indeed, the extent of the Veil's powers are not even fully understood, millennia later.
The last time magic was allowed to flow free, it was used as a weapon of war, monsters openly ruled over us, demons corrupted entire nations and we lived in fear of the things that prowled the wilds beyond our village walls. The Veil ended the reign of magic and started an age of reason. The supernatural world didn't go away, it just became far more subtle. The monsters began to wear human faces, speak with human tongues and live human lives.
And truth be told, most supernaturals are actually okay with how it all turned out, all things considered. Modern medicine, aeroplanes, cellphones, cinema and television, the Internet. It was a bumpy ride, but we're actually entering something of a golden age not seen since Atlantis fell. And so, the experiment of the supernatural pulling the strings of the mundane world is generally seen as a success. Your average supe doesn't really want to upend the system that has given them ice cream and skyscrapers for the "good old days" of the Iron Age just so they can openly reveal themselves as a vampire or a jinn.
And so the powers that be have agreed to uphold the Veil Treaty and maintain the separation between the magical and mundane world. They seek to minimize Veil breaches, help provide cover stories, and generally support the status quo.
Of course, this isn't a belief shared by all. While most of the supernatural world has signed on to the Veil Treaty, there are dozens of rogue organizations and factions, ranging in size from small cults to massive conspiracies, that would be all too happy to see the entire system come crashing down, and plunge Earth into another dark era. The Thule Society, the Knights of the Golden Circle, the White Lotus Society, the Order of Nine Angles, the Thirty-First House, the City of Brass, the cults of the Black Pharaoh, and the demons of the Abyss are just some of the more notable groups trying to cast the Veil down, so they may establish a new world order.
But so far, none have succeeded beyond small, local incidents. The Veil remains intact, and the world of magic remains obscured from the eyes of men.