The party’s descent into the void of the Demonweb Pits is slowed by webs below, and eventually they find themselves in the 67th layer of the Abyss. They land on the muddy shores of the Heaving Hills, where the River Styx flows by. Earthquakes roil the landscape, drowning the layer’s hills in the surrounding marsh and pushing up news ones to take their place.
As the companions crawl out of the muck and attempt to clean and dry themselves, Xenephon notices a herd of catoblepas drinking from the river. The big, hairy cow-like animals don't seem to pay any mind to the party's sudden appearance. But the thing about catoblepas is that they stink, like that of death mixed with swamp gas and skunk musk, giving the herd away as being much more ghastly than their appearance suggests. Everyone retches and gags at the smell, except Silversac Sam, who doesn't seem to notice it.
The half-orc and his companions make their way to higher ground to survey their surroundings. T'chon looks up, his thoughts of flying suddenly dashed by the sight of swarms of bloodflies above — swarms so thick they're like black clouds.
A moment later the aarakocra spots a ship moving along the river, a ship seemingly cobbled together from bones and body parts and chunks of metal and wood. A great grinning skull dominates the prow; the ship doesn't have any visible sails or oars, though. T'chon waves his arms and hails the ship, which responds by pulling up onto the shore of the river. A horde of tanar'ri pours off the deck, yipping and shouting and drawing their weapons. The companions ready themselves for a fight.
A woman then parts the crowd of demons. But this is no ordinary woman. Her pale skin contrasts with her black hair, hooves, and black bat-wings. Her eyes, like twin candles burning within black candles, fall upon Silversac Sam. With a soft voice that can barely be heard, she halts the advance of the other demons. They obey her and stand down. She introduces herself as Iku-Tursa “the eternal Tursa.” She and her twelve crewmembers are on their way to raid the devils in Baator. Their ship, the Corpsecutter, is bound by fiendish enchantments and run with the energy of crushed spirits, once a battleship in the Blood War but now used for raiding.
Iku-Tursa mistakes Silversac Sam for his father, Skarjaw Sam. After he corrects her, she is still impressed. Her demon crew cheers excitedly and rushes up to greet and meet him. The succubus invites the half-orc and his companions to join her. The tanar'ri are all eager to see if the son of Skarjaw Sam is as ruthless in battle as his father. They plan to raid Charybdis’s stronghold and could really use the party’s help. Feeling like they don't really have a choice, the companions agree to come aboard.
On their journey, Iku-Tursa takes a liking to Silversac Sam. She invites him into her private quarters, where she shows him all the carnal pleasures of the flesh. His dreams are haunted by visions of disaster: a giant cannon loaded onto the ship and laying havoc to an entire race of beings.
The morkoth called Charybdis lives inside a cave beside the River Styx on Avernus, the 1st layer of Baator. Getting there first involves navigating a narrow channel, then the treacherous rocks and whirlpools outside his cave. The entrance is partially flooded at low tide and completely flooded at high tide. The companions load onto a small landing craft with Iku-Tursa and a handful of tanari'ri raiders, and at just the right time, they make their way inside the cave. There, a barnacle-encrusted stronghold awaits. The walls appear to have been constructed from bones and pieces of ships of all sizes. The parapets are lined with devils, for Charybdis became aware of the new arrivals the moment they passed the narrow channel. Iku-Tursa planned on just charging up to the front gate, and nobody offers a better idea, so that's what they do.
After the battle in Charybdis’s stronghold, Iku-Tursa offers to take the party to a portal to Sigil.
The party exits the portal from Avernus through an arch in a park that’s abandoned and forgotton. The arch itself is made of blood iron, and the park is in the Lower Ward. Once statues of baatezu heroes stood here, but they are now in ruins and the park is overgrown with weeds.
T'chon, Xenephon, Krissy and Silversac Sam ask around for a way of getting back to Ysgard. Now that they have the true name of the Queen of Air and Darkness, the next step is to speak the name before Olidammara to break her charm on him. A tiefling tout directs them to a tavern called the Styx Oarsman.
Bryta leaves on her own to go check on Arctos Silveraxe at the Harmonium Headquarters. When she gets there, she learns that the werebear is actually on the run, an outcast from the Harmonium now. A dragonborn fighter named Tonat Shar has since replaced him as Factor here. When Bryta confronts him about it, he explains that Arctos was seen as being too negligent to be a Factor in the Harmonium. The werebear’s reckless abandon when assaulting the slaad in the 3rd layer of Ysgard left Sigil weak to demonic incursions. The Harmonium are already stretched thin; this unlawful use of resources was seen as unacceptable. Factol Sarin thus promoted Tonat to the position of Factor and put the dragonborn in charge of the City Barracks. Disappointed and angry, Arctos submitted his resignation to Sarin, who saw this as a betrayal of the faction and sentenced him to death. Nobody just quits the Harmonium — it is the law. Sarin also denied Baldur’s request for Harmonium reinforcements to Ysgard.
The Harmonium have since pulled out of Nidavellir so they can reinforce Sigil again. While they were gone, demons took the opportunity to flood into Sigil, so that now the City of Doors is more chaotic than neutral. Black storm clouds appear on the Outlands horizon…
Bryta is ordered to take a squad of Hardheads into the Hive Ward in search of slaad activity there. She takes them, but then makes up an excuse to split off, and goes in search of her companions.
Located in Sigil’s Lower Ward, the Styx Oarsman typifies the multiracial planar metropolis. The patrons run the gamut from good to evil, with the scales tipping toward the dark side of life. This inn caters to a mostly humanoid crowd. Humans, elves, dwarves, halflings, and githzerai compose the majority of the clientele; and then there are a few of those from or touched by the Lower Planes, including tieflings and even demons. The Styx Oarsman is a discreet location to meet and do business. And it is here the companions meet a cambion called Rule-of-Three who tells them how to get to Ysgard. The price is three potions. Once the potions are handed over, Rule-of-Three tells them that the portal to the first layer of Ysgard can be found in the Great Gymnasium in the Guildhall Ward, where the characters must simply jump into the cold pool of water while holding an ounce of silver.
A hulking bear of a man, wrapped in a tattered burlap cloak, catches Bryta's attention. A thick brown beard tumbles out from beneath his hood, and he grasps his ale mug in a gnarled, battle-scarred fist. The valkyrie leaves her friends' table, buys a couple of drinks from the bar, and sits down beside him at his table. He reveals himself as Arctos Silveraxe. Now a wanted criminal for leaving the Harmonium, he has no place in Sigil anymore. Bryta offers him a place among her companions, and he heartily accepts.
Krissy also notices someone she recognizes. Her ex-boyfriend, Vanthus Vanderboren, skulks in the shadows, watching the party. She advises her friends that they'd best leave now.
The first floor of the Gymnasium is dominated by the exercise field used for various martial arts and combat skills. The center of the field also sports three pools, one hot, one warm, and one cold. Beyond the field is an inner portico used for musical and other public artistic demonstrations and lectures. Past the portico are a number of separated rooms for music, dancing, sculpting, and other artistic endeavors as well as sealed and warded rooms for spellslingers to practice their arts without harming themselves or others in the process.
The companions go up to the cold pool and jump in. After submerging under the water, they resurface to find themselves on a snow-covered mountain slope in Ysgard.