I'm not Eberron conversant, so I hope my ignorance is not too problematic to understanding my post.
As I recall, a plane is bound apart from the Prime - it no longer orbits it. Perhaps when this plane was ripped out of its orbit and bound afar a bit of the Prime (or another plane?) was accidentally severred and now exists in a twilit fashion - not as far off as that other plane, but not really a part of the Prime either. Perhaps were it once existed is now shrouded in confusing mists where planar borders are weak, and due to existing co-tangent to its prior location, there is a chance that any that wander through those mists might find themselves on this demiplane ripped from the Prime. But perhaps whatever bounds the distant plane also makes it harder to return to the Prime from this severred piece of it, and the other plane somehow 'taints' this former part of the Prime - thus resulting in the various effects that collectively define being in Ravenloft.
Does this make any sense?