While we have a lot of planes in the Mystara cosmology, I was always fascinated with planar locations such as Sigil or the City of Brass. Here is an outline for such a planar location I am developing for my campaign right now.
Darkfire Castle
by Håvard
Darkfire Castle is a fortress constructed on the plane of Limbo. I am going with the assumption here that Limbo, The Plane of Shadow etc is the same plane, as developed in this thread. Feel free to make necessary changes if your Mystara has a different approach to these planes.
Centuries ago, a rift opened between the Plane of Limbo and the Dimension of Nightmares. It was discovered by a Glantrian Wizard, who was the Master of of the Secret Craft of Illusionism at the time. The Wizard discovered that the rift had been created by a strange fire-like form of energy, buring its way into the Plane of Limbo. The Wizard realized that this was a pure source of Blackflame. The Wizard contacted allies in the Five Shires as he realized the Hin were the only ones who could handle this.
Under the leadership of the Hin Master Mambur Tumblebrook, the Hin constructed fortress on top of the rift. Since then, these Hin have become permanent residents of the Plane of Limbo. Known as Halfling Shadow Walkers, these Hin are able to survive in Limbo. Living on this plane has extended their lifespans, but have also altered their appearance, making them lean and pale.
Darkfire Castle keeps a Blackflame Crucible, though it works slightly different from Crucibles that the Hin keep on the Prime Plane. The Crucible is protected by a Crucible Keeper and a large contigent of Shadow Walkers. These occasionally visit the Five Shires and Leehashire on Mystara in order to recruit new memberrs to their Order.
The most important thing to the Hin of Darkfire Castle is that their secret is never discovered. If the wrong forces realize that they are guarding a gate to the Dimension of Nightmares, that could be disasterous.