Book of the Damned
"A person is smart. People are dumb, panicky dangerous animals. Fifteen hundred years ago, everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know tomorrow." - Kay from Men in Black
The supernatural exists and always has. They walk amongst humans, camouflaged, and hiding their true selves. There are angels and demons, reapers and spirits, vampires, and werewolves. And many more. Only a few humans know of their existence, whether because they've seen them with their own eyes or live around them. The unknown can be explained, it can be seen, but is mostly ignored.