Stygia, the fifth layer is a freezing layer of cold and ice dominated by a murky ocean fed directly by the River Styx. The surface of the ice is covered with chilly marshes. The dark sky is constantly filled with lightning storms, and the ice may be lit by weird cold flames. Tantlin, the City of Ice, is built on a huge ice floe with a large harbor to the river Styx, and is ruled over by a huge pit fiend. The lack of any kind of law enforcement leads itself to gangs controlling much of the city.
Prince Levistus rules over this layer, frozen in a giant iceberg floating in the harbor as punishment for betraying Asmodeus. For many centuries, his realm was given over to the fiend Geryon. Geryon was recently deposed, and Levistus was reinstated, though he remains in his iceberg.
Godly realms on this layer include Sheyruushk, realm of sahuagin deity Sekolah, and Ankhwugaht, realm of Set.
Baatezu of this layer include amnizu, osyluths, and cornugons. In the water are giant octopuses, sharks, squids, whales, sahuagins, and scyllans.
Some also still serve Geryon, who lives now in his Citadel Coldsteel. There he broods with his minotaur worshippers and thinks of vengeance, but mostly he still does the bidding of the Dark Lord of Nessus, who holds the key to his restoration. Doing his bidding is the wolf-headed duke Amon, though some claim that Amon is not as loyal as he seems, and serves Levistus in certain matters as well.
Beneath the ice is Sheyruushk, the realm of the sahuagin deity, Sekolah. On another ice floe is the realm called the Steadfast Chill, where Kriesha (a goddess of Cerilia) dwells.
Ankhwugat is the realm of Set. Set is the dark master of the desert, lord of serpents, jackals, assassins, and creatures of the wild. Though dark waters flow underneath his realm, its surface is hot and dry as the sands of Pelion, and those familiar with the realm of Nephythys there will see Ankhwugat as a twisted reflection of that one. When Set and Nephythys divorced, Set went to Stygia to forget his pain. Perhaps once all Stygia was like Ankhwugat, and perhaps, if Set has his way, it will be again. Some claim that Set is mad and wishes to transform the layer or even turn his realm into a new outer plane set between Baator and Acheron. Others say he only wishes to reunite with his estranged wife Nephythys, and that the mad ones are the baatezu who try to earn his favor. The common chant says that Set and Levistus are headed toward a showdown. The will of a Lord of the Nine is supposed to be absolute in his layer, but Set has the good will of Ra and Levistus is imprisoned in ice.