5. Stygia
A black, icy ocean topped with ice and glaciers under an unforgiving black sky. Cold and darkness are omnipresent.
Geography
The "continents" of this infinite frozen ocean are massive glaciers, interconnected so closely that they cover the sea beneath almost completely. The ice is interrupted only by the similarly-lethal flow of the River Styx. Its oily surface winds across the glaciers without cutting deep enough to mingle with the ocean itself.
The glaciers are shaped into drifts and icebergs, but overall their flat surfaces offer no protection from the layer's bone-chilling winds. The ocean itself isn't much better.
The dark sea is barely warmer than the ice which tops it, and is inhabited by horrors which madden even the strongest minds. The sea churns, trapping swimmers below the surface with a malevolent will.
Occasionally, the ice floes support small amounts of moisture and earth, creating arctic swamps where tenatious but malevolent plants grow.
Ecosystem
Unlike other Hells, Stygia was almost entirely wilderness. The presence of devils was limited to their iceberg citadels and the Tomb of Levistus. Aside from these, what icy and acquatic creatures can survive Stygia's harsh environs have run of them.
Ecosystem Cycles
The black, starless sky occasionally flashes with sheet lightning, which creates a blinding brightness over the layer's ice.
Fauna & Flora
The few plants which live in the occasional arctic swamps are malevolent as the plane itself. Sharp, hard, and unburning, several species are vampiric, surviving off the blood of passing creatures rather than trying to process the icy waters.
As for fauna, several white Dragons make their home on the glaciers. Below the waves, things lurk which have precluded even the Archdevils from trying to conquer the depths. Some say they predate the fiends.