Stygia

Stygia was the fifth layer of the Nine Hells of Baator. A vast frozen sea dominated by ice floes and icebergs, it contained the divine realms of Set and Sekolah. It was named after the River Styx, which crisscrossed its entire extent. 
=== Geography ===
Stygia was a vast ocean of saltwater that was almost completely frozen. Its surface was almost entirely covered in ice floes and icebergs that crowded and completely covered the ocean. The only open body of water was the Styx, which kept its dark, oily water separated from the saltwater and meandered through the layer as if it were crossing a plain.
Some of the ice floes were large enough to support cities. Navigation between settlements was made relatively easy by the presence of the Styx. Small arctic plants grew in some locations, creating vast icy swamps that were slightly warmer than the rest of the layer. Those areas attracted several creatures and wildlife, but were at a larger risk of the thinner ice breaking.
The sky of Stygia was in a state of perpetual twilight and was constantly struck by lightning, which made it extremely dangerous for flying creatures. Thunder swept the land in a continuous rumble.
=== Government ===
The layer was ruled by the archduke Levistus, who was kept prisoner deep within a large iceberg known as the Tomb of Levistus in Tantlin's harbor. Levistus issued his commands and gathered information telepathically, as he was able to communicate with any other devil within 10 miles (16 kilometers). Levistus also controlled all of the amnizus, while he secretly planned to conquer another layer of Baator.
=== History ===
Before Levistus's rule, Stygia was ruled by the archdevil Geryon. After Levistus killed Asmodeus's consort Bensozia, he was imprisoned within the layer's ice for millennia, until Asmodeus ousted Geryon and transferred control of Stygia to Levistus. Asmodeus, however, did not grant Levistus freedom, but instead kept him within the ice and decreed that, as part of his punishment, Levistus must offer escape and safety to desperate individuals. Able to conduct all his affairs from within the ice, Levistus then became expert at trading souls of the prosecuted and criminals in exchange for a quick escape.
=== Rumors & Legends ===
Stygia was rumored to have been originally a world in the Prime Material plane that was doomed to destruction. Its inhabitants then pledged their entire world and their souls to Asmodeus in exchange for safety, which resulted in their world becoming one of the Hells. This hypothesis was based on the unusual variety of creatures native to the Material Plane that inhabited the layer, although no more decisive evidence had ever been found to support it. It remained a topic of speculation whether the original world's riches were still buried under the ice.
The layer was also rumored to be the headwaters of the Styx, both due to the river's ubiquitous presence throughout the landscape and to the strength of its memory-draining effects there, which were capable of erasing an individual's soul.
=== Notable Locations ===
Ankhwugaht, realm of Set.It was inhabited by numerous liches and mummies, who occasionally left the realm to trade with the other layer's inhabitants.
Sheyruushk, realm of Sekolah, located below the frozen surface.
Tantlin, the City of Ice, largest city of the layer. It was built atop an ice floe on one of the banks of the Styx and was ruled by an unknown pit fiend.
=== Inhabitants ===
In addition to the wildlife that inhabited the layer, tribes of frost giants also wandered across the ice with no fear of the devils.
The diabolic population of the layer included abishai, amnizus, erinyes, gelugons, and spinagons. Also uniquely among the layers of Baator, pit fiends were exceedingly uncommon in Stygia. Hamatulas and succubi were also commonly found.
Stygia was unique among the layers of Baator in that it was almost entirely untamed and wild, with no significant part of its territory devoted to any particular purpose. For that reason, its wilderness was inhabited by several aggressive creatures such as dire wolves, frost worms, krakens, mammoths, polar bears, sharks, and remorhazes, which were used as practice for devils in military training.
Stygia under Levistus:
With Levistus in control of Stygia, the harsh, chilling realm became even more unbearable, with a sense of starvation and desperation permeating the denizens and landscape. 
Prison of Levistus:
Prince Levistus's tomb aimlessly floated throughout the freezing ocean, occasionally getting stuck between larger icebergs at the outer reaches of the plane for years at a time and sometimes passing in and out of the River Styx. Levistus was incapable of controlling the direction in which the iceberg moved despite his regular attempts to do so, such as by having sails and rudders attached, and it was said that when the currents caused it to drift in an undesirable path, the light laughter of Asmodeus could be heard. 
Erinyes keeps the tomb safe from above while ice devils on ice floe rafts protected it from below, although the gelugons were known to take bribes from visitors wishing to see the Prince of Stygia.
The enchanted ice encasing Levistus was incredibly powerful and ancient even by the standards of the baatezu. Even if the snow and sleet of Stygia didn't freeze over most attempts to break through it twice as fast as the devils mined, its resistance to magic could best any attempts to summon and teleport Levistus out, with even wish and miracle magic failing. Only persistent destruction of the ice or melting it with fire, magical or otherwise, could permanently alter its shape, but even then it almost immediately regenerated any attempts to free Levistus specifically. There was a magical method by which souls sacrificed to Levistus would empower him and melt the ice and, but only good-aligned beings of fire would cause any significant change. Only Asmodeus seemed to be capable of freeing Levistus from his tomb, and only did so once a year so that he could sulkily attend the meetings of the archdevils in Nessus.
Despite this, other areas of the iceberg were able to be hollowed out, and had been converted into a labyrinthine array of corridors, chambers and vaults filled with ice devil militia and wary guard posts. The tunnels wound upwards and the prison became more akin to a fearsome palace of ice the higher one rose, not that most intruders would be allowed to see it before being caught. The various properties of the ice allowed the guardians to more easily spy on trespassers, seeing perfectly for hundreds of feet in some clearer areas while using the hairline cracks in certain fissures to listen to distorted sounds from afar. Perhaps the biggest threat of the tomb was its sheer cold, preserving corpses of intruders perfectly and using wicked magic to keep mortals alive. Often times, imprisonment in such areas, known as Halls of Frozen Memory, was a punishment for heroic trespassers and typically their would-be rescuers, as well as disloyal or traitorous devil servants.
Those trapped were as sentient as Levistus but had likely gone mad, filling the area with insane, inescapable screams and nonsense, with the rare intelligent soul being unable to converse with others for too long lest the stillness of the new soul cause them to be entrapped as well. Souls were delivered through docks anchored to the outside of the tomb and suffered a similar fate once pressed inside through monitored ramps, forced to keep moving throughout the halls or else become frozen, although many were already mad. Devils throughout the Nine Hells occasionally schemed to steal from the vaults of Levistus, an idea the Frozen Prince encouraged since it netted him more souls, although none had ever succeeded in stealing from his vaults and devils trapped within had to escape or else be hunted by non-devil teams and sentenced to a fate worse than death.
Guests deemed worthy to be invited inside the palace's suites were met with masterfully crafted decorations and furniture of crystalline ice. Accommodations were both piercingly cold in order to keep the archdevil's presence in the minds of those present and fanciful to remind visitors of his class and riches. Offering flesh and warmth to the chilled visitors, Levistus's succubi servants reported any important information they discovered, assuming visitors were being allowed to talk to him and not an intermediary. After scaling a spiral staircase, Levistus could be found 100 ft (30 m) within the peak of the iceberg inside a towering rampart, leaving anyone trying to speak with him exposed to the Stygian winds. The Frozen Prince could be seen across a perilous 60 ft (18 m) gulf with flying devils spiraling overhead, accompanied by an ancient white dragon.