The Bottomless Pit of Chaos and EvilEdit
There are multitudinous layers of the Abyss, perhaps even an infinite number. No one has ever catalogued them all, and it's doubtful that any of the plane's natives, let alone any other bean counter, has ever considered the effort worth the time and danger. Plain and simple, no one's ever done so because each layer is so horribly grotesque that a berk would have to be barmy to want to see any of them. Even fewer sods have the power to actually do so and survive. For one thign, almost every layer of the Abyss is completely inhospitable. For another, this plane is home to the tanar'ri, one of the two races locked forever in the infernal Blood War—and they aren't particularly hospitable either.
Watch out for those tanar'ri bashers. They'll either try to impress a berk into one of their armies—whether for the Blood War or a local squabble—or they'll flat out put him in the dead-book. Their reason? As often as not, just because they can. Don't expect a better reason for beings of utterly evil chaos. They're devoted to siezing everything they can for themselves, and killing or enslaving everyone else. To their minds, that's the only way to power, and power is the goal of existence.
Abyssal Locations in SigilEdit
Lake of Fire (Plain of Infinite Portals)
Abyss Central (Plain of Infinite Portals)
Demon Fortress (Plain of Infinite Portals)
Devil Camp (Plain of Infinite Portals
Abyssal Mount (unknown plane)