The Mists

The Mists are the binding force of the universe. They were there before everything else and they are the source of all things including good and evil.

The Mists contain and connect the mortal worlds and The Rift, the worlds of the immortal gods and spirits.

Some immortal beings are apparently able to travel directly through the mist to step over from one world to the other, while mortal beings have to use special portals, such as the one located in The Tomb of Primeval Kings.

Trivia
The Mists seem to be similar to what æther (the "5th element") is in classical alchemy, natural philosophy, and - in modified form - in religion and esoterics.