666 Layers of the Abyss
There are as many different descriptions of the Abyss as there are souls in all the known planes of existence. To some it is a region of overwhelmingly fierce desert sandstorms. To others, it is a realm of explosively unstable volcanic activity, boiling lava, and molten rock. Still others find it to be a place blinding snowstorms and sub-zero arctic glaciers.
In truth, if there ever can be reliable "truth" to come from such a place, there are multiple layers of the Abyss, each ruled by a demon lord or lady. The most common means of transportation between these layers is through the use of portals scattered throughout the realm. Often these magic portals allow access in only one direction, leaving the poor traveler stranded beyond the portal and at the mercy of whatever lies on the other side.
One should be prepared to face any number of godlike fiends who control legions of foul creatures, each able to destroy an entire village. Some demon lords are known to be immune to particular kinds of normal and even magic weapons. Most demons are able to go into fits of rage and use the fighting styles of warriors, while others rely on powerful magic or breath attacks. One should always use caution when battling a demon prince or lord, and when visiting this plane in general.