The Abyss

The Abyss is the plane of existence where demons are born. A chaotic universe unto itself with almost infinite depth, its physical space has more in common with a vortex than an actual solid plane.
The actual layers of the Abyss itself is as varied as any climate on any plane. Boiling rivers of chaotic energy, black waters with souls and creatures lurking below the surface, spires of civilizations and ages past, blizzards making way for burning deserts.
The Abyss is the most unstable of all of the planes, rips and tears appearing at seemingly random allowing its denizens to escape into whatever plane the holes eject them into. While exceedingly rare, there have been accounts of demons tearing their way into the streets of cities, bringing with them the pandemonium one might expect.
It in itself is thoroughly unexplored, only several dozen layers have actually been excavated through, with an estimation of nearly 200 known layers. There is only one thing for certain though: although all demons are born within the Abyss, every single one of them wants to escape it. Why? Your guess.