Dark and foreboding, tangible yet wispy, The Tenebrae is a realm of the Void that overlaps all of Creation. It is not merely a place of shadows; it IS shadow. Indeed, were it not for The Tenebrae, the world would be without shadow at all, and light and darkness would cancel each other in stark and defined lines. Without The Tenebrae, colors would be unbearably bright, lines painfully crisp, and darkness would be menacing and lethal.
For the vast majority of people, The Tenebrae is only a theory, a place pontificated by sages and divines. For the common man, shadow is what the suns casts across one's lawn and shade a welcome relief from the heat of the day. But for others, The Tenebrae is far more. For the "shadow walkers," it is a destination and a journey unto itself. But even among those who know who to get there, very few are willing to brave the experience.
Shadow walking is the only known method slipping through the wispy barrier between the material world and The Tenebrae, and it requires a great deal of patience and practice to learn it. Many do not survive the training, for shadow walking is inherently dangerous. The Tenebrae is populated by distorted and mangled shadow versions of real creatures, known collectively as "Grotesques."
Glowing white eyes amid shifting forms of tangible shadow--called tenebrae (lower case)--hissing and skulking about, Grotesques feed off the souls of living creatures. They are instantly drawn to the presence of life; indeed, they can sense it through the diaphanous barrier where The Tenebrae touches the world, and they long to breach the divide and feast on the living. These Grotesques make travel across The Tenebrae necessarily a brief and harrowing experience. Even the most experienced shadow walker limits his exposure to only a few seconds.
Why do it, then? Why disappear into the shade of an elm tree to reappear moments later in the dimly lit corner of the King's bedroom? Think about that for a second...
Thieves and assassins comprise almost the entirety of shadow walkers, and the most infamous of these are the Skaen Sisters, a sisterhood of the Nephreqin. So deadly and feared are these women that even the Nephreqin ascetics are known to keep their houses either brightly lit or completely dark. A more mercenary reason to risk it all is the possibility of harvesting raw tenebrae, the tangible substance that makes up most of that realm. Theories abound that raw tenebrae can be converted into physical things and put to wonderful and powerful uses in the material world. Weapons immediately come to mind.
The only known way to prevent shadow walking is to remove shadows. A shadow must be large enough to engulf the shadow walker, and it must be an actual shadow. Total darkness is absent shadows just as much as total light. If a room is brightly lit--or if it is pitch black--the shadow walker is trapped, either in the material world or in The Tenebrae.
And being trapped in The Tenebrae is a certain, immediate, and gruesome death sentence.