In this day an age, most believe that the Shadow Company and Shadowlock Keep grew up together. Some will tell you that the SC built the keep. Others will say the SC gots its name from SLK.
The fact of the matter is that SLK existed far longer than the SC, and the SC recieved its name when it was being formed back in Xen’drik. It just so happened that Breland was having difficulties with the tribes in the west and the SC had just arrived in Khorvaire.
SLK was built in the days long before the five nations were forged into a single empire. The site of the SLK is a manifest zone between Eberron and the Plane of Shadow. People in the area routinely went missing and strange creatures that seemed to be made of Shadow would occassionally roam the area.
In Year XX, the king of Breland (look up ancient name) and a Shadowdragon that went by the name of Dusk met to deal with this situation. Both wished to prevent this leakage into each other’s realm. Agreements were struck and SLK was built. Dusk took up residence in a huge cavern below the keep that allowed a crossover between from Material to Shadow. A Dimensional Seal was built and bound to two rods, one controlled by Dusk and one by the commander of the SLK. Both rods must be present and commanded to allow creatures from either the Material or Shadow to cross the boundary. Those that do cross may cross back to their native plane within the overall manifest zone (a radius of approximately 3 miles from SLK).
The commander can only be named in a ceremony led by the rightful leader of Breland. The commander may call upon Dusk to help defend the Keep, but Dusk will only do so if she feels the Keep is truly threatened. She has no other concerns of mortal affairs, and her interest in the Material Plane ends beyond the manifest zone.
Life inside the keep can be rough. The area is so intertwined with the Plane of Shadow that it plays tricks on one’s mind. The brightest of lights appear dim in the walls of the keep (all light sources are 1/2 their normal range). Shadows flickers are exaggerated and are deceptive (-2 to spot checks for those who have spent less than 1 month within the keep’s walls). Even those with Darkvision report the same issues with their vision. It is also written that those that have crossed over to the Plane of Shadow here have great difficultly distingishing that they are not on the Material plane any longer (ie, the Material plane is as intrusive into the Plane of Shadow as the Plane of Shadow is into the Material).