Nightwalkers - Death Walking Terror

To see a Nightwalker is to see death incarnate. People who witness one speak of it's height, about 20 ft tall roughly , made of living solid shadow, with no discernible features and two horns sprouting from it's forehead. It seems to be made in part of the Shadowfell, and it surrounds itself with power from this dimension. The creature is born when mortals try to summon the Shadowfell itself into the Material Plane, or at least some creature or portal from it. If it fails, a Nightwalker is sent from the powers that govern the Shadowfell. This response is to stop others from trying to do the same for it does not tolerate being abused in such a fashion. The Nightwalker will proceed to wipe out all intelligent life in the region it forms to remove anyone trying the same, learning of the Shadowfell, or of the rituals needed. In time it will fade away but when this happens it is a threat to all life. It is made up of the stuff of the Shadowfell, and of those that it has either killed before, or those that died during the ritual.