Few names evoke as many dramatic and terrifying images in the minds of mortals as The Nine Hells, and with good reason. Baator personifies the epitome of the Lawful Evil ethos as a place of punishment, oppression, and endless, soul-crushing conformity. It is the eternal destination for those who force their will through unjust violence, tyrants, cruel overseers, malicious judges, and unscrupulous barristers and politicians.
Just as Dante's Inferno, Baator is arranged in nine layers. Each one is ruled by an Archduke, a unique devil that is granted their status by Asmodeus. While the Archdukes aren't deities themselves, they manage to use various methods to alter their layers to their tastes and necessities. There are pacts with Lawful Evil gods, the use of slaves taken from across the entire cosmos, and powerful magic that is the envy of every wizard in Acaelica.
Getting from one layer to the next is not physically difficult. There are often paths that can be walked from one layer to the next (and flight can be invaluable). One must take care, however, in avoiding the guards, patrols, etc. that are meant to keep devils, petitioners and outsiders in their proper place.
Aside from the cities and strongholds of the devils, there are a few other unique landmarks. Tiamat and others have their own personal domains among the layers. The City of Dis actually ranks as one of the great planar markets, as devils have great need of armaments and war material for the Blood War.