The Dead Lands of Athas
The Black is one of two planes that directly touch the prime world of Athas; the other is the Gray. The Black, a realm of chill darkness similar to the Demiplane of Shadow, can be accessed wherever light creates shadows. In the absence of light, no connection to the Black exists.
This vast dimension of darkness separates everything that exists from everything that doesn’t. It’s an absence filled with numbing cold, endless darkness, and even living beings.
The Shadow People, who some erroneously refer to as shadow giants, live as part of the Black. These one-time halflings were the loyal servants of Rajaat the First Sorcerer. To complete their betrayal of Rajaat, the Champions trapped the halflings in the Black, and now the halflings can only interact with the world through shadows. The Black lurks below the surface of all things, like the leathery shell of a great egg, buried shallow and about to hatch. The world of Athas exists outside the shell of this egg. Inside, cold gloom fills the plane, the entirety of which is a dark stain visible only as an absence.
Shadow wizards tap into the Black to gain the energy needed for their magic. Indeed, those who follow shadow sacrifice a portion of themselves to the Black, becoming part shadow. They depend as much on light as they do darkness for the power they wield.