Phlegethos
The fourth layer of the Nine Hells consists of rivers of magma flowing between volcanos. There are many small communities gathered up in various caves where they try to hide from the dangers around them. The air here is heavy and burning. The smell of ashes and hints of sulfur are ever-present. The sky is only clouds of ash, lit from below by the fires.
The only true city is Abriymoch, which is a pleasure palace of sorts, but the city is also the location of the largest court in the Hells. Many wish to go here, believing it to be a relief from the torment elsewhere. In reality, every bit of pleasure is paid for by taking on so much worse. And for many, the only hope of ever getting rid of a tiny bit of that debt is to sell pleasure to others.
While the formal ruler of Phlegethos is Fierna, it is a duty shared with Belial. The two are thought to be mortal enemies, tentative allies, or close lovers. Maybe all three. they are also said to be mother and son, father and daughter, siblings, husband and wife, neither, or several of these.
Geography
To learn to navigate the plane is to learn how the magma flows. To survive navigating the plane is to learn how to avoid the magma. And for many, the long-term strategy is to seek out the city of Abriymoch, hoping that the dangers of those living there are less than the monsters and devils roaming the landscape.
Fauna & Flora
The species thriving here are few, but extremely hardy. But they can be nutritious enough to sustain those trying to survive in the mountains as long as you don't mind food that tastes of sulfur and charcoal — or competing with nightmarish creatures for it.
All in all, the plane does its best to convince everyone on it that they need more to feel that they want to live and not just can live.
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