The Shadowfell

THE DARKER MIRROR OF THE MATERIAL REALM
A dark mirror of the material world, the Shadowfell surfaced as the chaotic and tainted mirror of all the deities created for their mortal children. It known as being the first place spirits reach as they begin their journeys towards either heaven or hell.

It apparently contains realms such as the Village of the Barovia, with entire peoples and cultures within.

The Village of Barovia
Ruled by the vampire Count Strahd von Zarich. The citizens live under squalid and poor conditions but continue on with their routines with no regards towards their past or their futures. The mists that fill the land act as active power over the realm and enable Shadow Crossings that connect this realm; it is done with the intention that someone will be unfortunate enough to cross over.

The mists themselves can be affected by the interloping of other powers such as when the Cult of the Chainless that resided in the Village of Barovia managed to open a rift into the Far Realm beneath the Durst Estate. The powers of the far edges of the universe granted the house sentience and magic such that it continued the imperative that its previous owners had: continuing their sacrifices to both the forgotten Chained Oblivion and to Strahd as well.

The latter was not pleased with the invasion of new powers into his realm and culled the cult, but the house’s machinations continued as the count himself had not the power to seal the rift.