Some call it the Elemental Plane of Darkness, the Shadowland is the Spiritual Plane which powers black magic much as the Elemental Planes power arcane magic. Not much is known about the Shadowland. No mortal has ever been able to enter this world. Each time a mage attempted to enter the Shadowland in the recorded history of our world, the Portal was closed shut before it could have properly opened. There are indications that the Portals were closed shut by the powerful creatures that inhabit this world, although of course there is no proof, since noone has been able to enter this world and confirm their existence. It is however plausible to assume that they exist, since creature exist in the Planes that power arcane magic as well. The haunting   
question, rather, is why these creatures insist on keeping mortals out of their world. If they are powerful enough to do so, they certainly need not fear any mortal intruders. Some fear that these beings are planning something behind closed doors, and by the the time we learn of what that is it may already be far too late
. - Excerpt from the treatise entitled Arghard and the Multiverse, written by the orc mage Azrahl roughly two centuries before Twilightfall