Disclaimer: While Primoris is an original character, meant to be the first and most powerful nightmare ever (basically a manifestation of primal fear and darkness), the universe he inhabits is that of Neil Gaiman's Sandman universe. I don't own that.
When the first sentient creature ever created, all alone in the universe, closed its eyes and felt first nothing and then fear, I was there.
When they, and others like them, reproduced, I was still there, growing stronger, and darker, and greater, and more terrible.
When the first prey feared for its survival, dreamed of being caught, and killed, and eaten, I was the predator. When the predators feared bigger predators, and the terrible emptiness of drought and famine, I was the teeth and claws and fur and rush of warm blood, the biting frost and the cracked land.
And when the first humans dreamed, I was the dark, shapeless, amorphous terror, the predator, the evil they fled without knowing why but with the certainty that if they did not run something horrible would happen, and they were right, for you can die in dreams. They lit fires in cave entrances to scare away the wild beats, but the flickering firelight cast shadows on the walls, and shadows are insubstantial and darkness and cannot be speared, and the humans feared them worse than the beasts. And when they finally dared close their eyes against the dancing, shifting shadows, they saw it again, and they screamed.
I am shadow, and air, and vacuum, and darkness, and blinding light, and death, and nothingness, and everything crowding in upon you, and chaos, and cold structured order, and most of all I am fear and terror and the nameless shadow on the wall.
You use electric lights now, and they are brighter and harsher, but the shadows are still there, and they are not always on the wall; they have moved, flowing, shifting, to your mind. And I am still here.