The Shadowfell

Sometimes referred to by planar travelers as Shadow Orr or the Gray World, the Shadowfell is a dark reflection of the material world. Hypothesized to be one of the splinter universes created after the fall of the heaven-dragon when time flowed in every direction, the Shadowfell appears to be a world where the mortal races perished or were wiped out long ago. This plane is older than Orr and bares innumerable ruins and catacombs, some of which appear to correspond to the locations of cities and sites in our own dimension. Even the Sol of that world has nearly burnt out and its faint orange glow barely warms the earth, its light casting only enough sputtering rays to provide safety from the dark dwelling denizens of the plane for a handful of hours during any given day. The Shadowfell is inhabited by dark spirits of undeath, filth, disease and malice. It is a Plane where primal fears are embodied, where demons walk unabashed and where the animated dead shamble in hordes.
Travelers to the Shadowfell speak of strange metals found here and nonfunctioning, rusted machinery reminiscent of the marvels of Drin. They speak of a world torn asunder and blasted apart from some unknown, devastating and apocalyptic power. Perhaps most curious of their findings is how the monsters of the Shadowfell hunt for magic and target magic-users who wander into the dimension with beastial and slavering ferocity. The Shadowfell is strangely devoid of the winds of magic. Arcane power seems to barely trickle into it, making it difficult for sorcerers and wizards to make use of their talents and spells. When magic is cast in that plane it acts almost like a beacon or a powerful lure, drawing out whatever horrors lie for miles around. Even the spirits of the dead, presumably those that once belonged to the civilizations where now only ruins lie, are driven into hateful frenzy at the appearance of Arcane power within the dimension. Whatever catastrophe brought the Shadowfell to ruin seems related to this vengeance on magic but we of the material plane can only speculate the circumstances.