Baator

From Manuel of the Planes (3e):

It is Hell, where sympathy withers and malice blooms.

It is home to devils.

It is the plane where the rule of law reveals evil's heart.

The Nine Hells of Baator, sometimes simply called Baator or even Hell, best satisfy the imagination of travelers, the greed of treasure-seekers, and the battle-fury of paladins. It is the ultimate plane of law and evil, the epitome of premeditated, crafted cruelty. The devils of the Nine Hells all obey a higher law than themselves, but all that really means is that they chafe and rebel in their caste. Most undertake any plot or action, no matter how foul, to advance themselves. At the very top of the hierarchy is Asmodeus, who has yet to be bested. That is the law of the Nine Hells.

The Nine Hells compete with any other lower plane for their sheer diversity of vileness. The devils are more cunning, more subtle, and more dangerous than other fiends—or so say the devils. A demon revels in slavering, insane, evil power, but a devil always has an agenda, a plan of attack, and a carefully conceived plot for retribution if necessary.

Baator consists of nine layers, each lower than the next, like ledges stepping down into an ever-deeper pit. Each layer descended gives a traveler a better view of the Nine Hells as a whole; the layers fit together like puzzle pieces, and each new descent allows a traveler greater understanding of how the puzzle comes together. It is an evil enticement.

The Nine Hells are home to minor, major, and noble devils, as well as true deities of evil (such as Kurtulmak of the kobolds and Sekolah of the sahuagin). The Dark Eight are eight powerful pit fiends who control the primary devil armies engaged in the Blood War. Worse still are the Lords of the Nine: Each lord controls one layer of the Nine Hells, hovering in status somewhere between scion pit fiend and demi-deity. Of course, all the lords ultimately answer to Asmodeus, the Lord of the Nine, who rules from Nessus, the bottom layer of the Nine Hells.

The layers of Baator are nine, and each layer has its own ruler. All other rulers answer to the lowest, Asmodeus, in the Pit. Each of the nine layers has its own unique environment, but they're all inhospitable and possibly outright deadly. Travelers to the Nine Hells had best know how to get back out, lest they are waylaid by a devilish press gang bound for the Blood War. But even a small chance of survival in battle would be preferable to enslavement in the  Nine Hells for eternity.

The politics of the Nine Hells have rocked back and forth over the millennia, often accomplished by artful rhetoric or subtle poison, but sometimes accomplished by outright conflict involving armies of devils warring against each other. The city of Dis on the second layer has withstood siege more than once at the hands of aggressive devils. However, in all the time that Asmodeus has ruled from below, no revolt has ever succeeded in replacing the lord of the ninth layer with a different devil, despite a ploy designed by Asmodeus that once allowed his rivals to think he had been dethroned.

Though the Nine Hells is governed by devils, some sages believe that the devils commandeered the Nine Hells from a far older, stranger race now simply called ancient baatorians. It's possible that remnants of this mysterious race still inhabit isolated portions of the Nine Hells.