The Nightmare of Sigil
When night comes, Sigil shows its literal darker side. The Crossing, commonly referred to as the 'Thirteenth Hour', occurs every night without fail and plunges the city into the depths of the Abyss. Come midnight, the bleak and cloudy air is replaced by choking, raw abyss, and the island is surrounded by the environment of one of thirteen abyssal realms.
Preparations for the Crossing
The truth is there's nothing that can really be done about a Crossing, it's been an inescapable fate ever since anyone can really remember in Sigil's history. However, there are steps that can be taken to ensure the best chance of survival.
First, and most obvious, is to return home long before a Crossing. At 6pm every night, the Abyssal Clocktowers ring with that evening's destination, and ring again at 11 as the final hour warning. City curfew is recommended to be 10:30pm to avoid being outside when the bells ring. Some major streets have temporary shelters for those caught too far from home, and much of the city will welcome stragglers to their homes for the night (though no one will turn down the chance to earn coin out of it). Some have even been desperate enough for safety that they've retreated into the sewers for the night, though such actions are extremely ill advised.
Second, the city treasures a substance known as incense. Named due to its thick, pungent smell, incense is found to repel demons and evil intentions alike, while preventing the manifestation of Sin simply by exposure to the abyss. Making incense is a careful art that very few can do right, as the ingredients are rather particular, and the process itself requires an alchemist's hand. Incense comes in a few varieties, scents differ and as does their effectiveness, based on what ingredients are used and how.
For those planning on venturing into the abyss for resources or other reasons, it is well advised to bring plenty of incense, have weapons on hand, and travel with companions.
The Abyssal Clocktowers
While the city has clocks and clocktowers scattered throughout its entirety, there are thirteen glorified towers known as the Abyssal Clocktowers. East to point out due to their elegant, dark metal designs and intricate multi-faced clocks, the Abyssal Clocktowers are spaced in such a way that their haunting bells can be heard in every corner of the city. They each are surrounded by a plaza so the masses can watch for the predicted crossing.
Many urban legends surround these towers, from their creation to what they hold within as no doors are built into the walls. They've been established for nearly as long as the crossing has been a threat, or so its been said.
Each tower is built with four walls, clocks on each. Each wall has one regular 12 hour clock face, one 12 hour clock face (the realm face), and a ten hour clock face to measure the general danger the city will be in that given night (the danger face).
The Thirteen Abyssal Realms
The one saving grace to the Crossing is that its semi-predictable. Every crossing, the city ends up in one of thirteen set realms within the abyss, each with its own environment and portfolio. When the Abyssal Clocktowers display that night's destination, the city has enough information to be prepared for the night.
(Realm Name – Threat Range – Description)
(*) notates a realm that contains collectable resources
Undersump, The Abyssal Wastes – (1-2)* – The sewers of the Abyss, the Undersump is a collective garbage heap of scattered remains from other parts of the Abyss. The entire realm is toxic and riddles with oozes (which is believed to be the origin of the native oozes in Sigil), but otherwise the ideal realm in a Crossing due to its relative non-threatening environment. It is advised you have extra incense on hand, as the waves of toxic air will burn through incense much faster. The Demon Lord here is known as Jubilex, though there seem to be no known written encounters with him.
Ghahazi, The Ruined City – (2-4) - A massive, demolished city with buildings from every era and region. There are multiple, expansive catacombs spanning below the city, and are believed to lead to grand treasure. However, it is easy to get lost very quickly, and unable to return before the hour is up. It appears the only active threat in the city are the demonic gargoyles that live among the buildings, but they don't usually attack unless trespassing on their territories. The residing Demon Lord is Xoveron, the four headed four armed gargoyle lord.
Midnight Isles, the Isles of Sin – (3-5)* – Deceptively peaceful in appearance, the Midnight Isles are a series of dark but beautiful islands among a still pitch black sea, with an ever present full moon in the cloudless night sky. Each island is associated with a powerful entity that has been assassinated by the Demon Lord Nocticula and a new island grows for each new assassination, expanding her realm. Every island has its own ruler: a unique succubus or incubus; each island also has its own ecology based on the victim the island represents. Nocticula herself resides on the largest, centermost island known as Alinythia.
Cathedral Thelemic, The Deviant Palace - (4-6) – The city-sized palace known as Cathedral Thelemic is a massive establishment of taboo and deviant actions. Every room is said to hold an act of tempting desires, and while not obviously dangerous up front, most of the beings in this realm are overbearingly tempting to lead others away from Sigil. Demon Lord Socothbenoth is notorious for his perversion and ability to provide any desire, and as such it is advised to have plenty of incense and charms on one's person.
Charnelhome, the Mansion of the Mire - (8-9) – One of the least ideal realms to end up in, Charnelhome is a massive mansion built upon a bogged swamp filled with carnivorous plants and hidden beasts, and a full moon is always present. The mansion itself is said to be most welcoming to enter, but leaving means traversing countless traps and rooms of torture designed to keep prisoners alive as long as possible but break them in every way. The Demon Lord residing, Shax, is very frequently encountered due to his nature of wishing to experience the torture of his guests first hand.
Blood Clefts, the Barren Wastes - (7-8) – What could (with imagination) be seen as a relatively normal environment, The Blood Clefts are a vast expanse of plains, rivers and hills. However, the realm is entirely devoid of vegetation, the rivers are black or dried up entirely, and deep chasms run through the land, with the entire ecology stained blood red (even the sky). Few creatures live her, but those that do are obscenely dangerous, hand picked followers of the Demon Lord Areshkagal herself.
Uligor, The Eternal War - (2-7) – Uligor is a bit of a mess, where most of the other realms are environmentally predictable, Uligor's patchwork of ecology makes nearly every visit unique. It's very difficult to even predict the threat here, as on good days the Crossing be neigh uneventful. Uligor is a realm ravaged by eternal war, set forth as amusement by the Demon Lord Orcus. Vast armies of undead and demonic titans clash daily, for years on end, and it's believe that those who are lost or killed here are added to the armies to fight eternally.
Pleroma, The False Paradise - (6-8)* – Countless writings of this realm describe it differently, for the False Paradise takes on illusory appearances most appealing to the eyes of the beholder. The only consistent detail is the gorgeous spiral architecture of the city Diovengia, the capital at the center of the realm. Rarely, Sigil will appear in or nearby the capital, granting possible access to the real attraction here: The Forbidden Library, Scrivenbough. Demon Lord Abraxas is known as the lord of forbidden lore due to his possession of secret knowledge. Only a handful of successful expeditions to the library have been made, one of which resulted in the knowledge of how to create gearforged.
Kurnugia, The Abyssal Wilds - (8-9)* – Hills, forests, rivers, and oceans, Kurnugia is the closest thing to real-world ecology complete with a clear full moonlit sky, fresh water, bountiful food, and plenty of creatures. Plenty. Kurnugia is also named the Realm of Monsters for very good reason. The Demon Lord Lamashtu, mother of monsters, resides here and keeps her realm populated with her children. The closer to Lamashtu's presence Sigil is, the more likely the real threat becomes apparent. Lamashtu, mother of monsters, is also a perpetuator of nightmares. She is rarely actually encountered, but her radius of madness inducing nightmares expands for miles around her.
Shibaxet, The Rooks – (5-7) – Without fail, Sigil will always arrive on the edge of this precarious precipice, a chasm that drops endlessly into the abyss. The clear, semi-lit sky always has a full moon present, even though the sky gives off enough light to be mid-evening. Winged beasts that nest among the cliff walls will raid Sigil's street's endlessly, but are known to be less interested when people are indoors. Demon Lord Pazuzu resides within a city hidden among the cliffs, and emerges to raid the city himself if too much attention is drawn to it.
Barren Wood, the Dead Forest – (4-7) – The cold and foggy Barren Wood is an endless forest of dead (or dying) coniferous trees, with patches of grey snow in the clearings. Broken and run down houses lay scattered around the forest, some gathered in makeshift villages, but most are empty. Still, one must approach with great caution, for the witch hags that make their home in this realm tend to hole themselves in these houses and wait to lead wanderers astray. The Demon Lord Mestama herself is a hag, one of unending cruelty, and welcomes wanderers with cruelty in their hearts to remain and be nurtured within the woods.
Ivory Labyrinth, the Abyssal Maze - (4-6)* – A massive, city sized labyrinth made of shimmering, polished ivory, is a rather stark change of appearance from the rest of the city. Sigil appears within the labyrinth itself, in a new location every time, and it is highly advised one does not go out of eyesight of the city. Still, the walls of the Labyrinth are overgrown with fruit bearing trees, always growing just out of reach of Sigil, forcing people to go further than comfortable to harvest them under an eerie full moonlit sky. The Demon Lord Baphomet is said to perpetually roam the maze itself, and can be heard in the distance, but is rarely ever encountered.
The Winding Wood – (10)* – No other name is given to the Winding Wood, and is by far the most feared of the realms. The majority of the realm is a dark, gloomy forest with such heavy air that individuals find themselves saddened, tired or depressed just by exposure. Occasionally, Sigil will appear in a region where the main attaction is visible: The Ticking House, a large clocktower workshop that houses countless enchanted inventions. Shivaska, the Demon Lord here, defends the Ticking House with a violent protection, housing hundreds of vile, terrifying beasts within. Shivaska is rather fond of prisons and imprisonment, especially of children, and these nights particularly it is advisable that children are kept close.
A being that dies on its own plane is dead. A being that dies on another plane returns to its own. So what happens in Sigil as it hops between the Abyss?
Sigil is defined as Material Plane even when within the Abyss. It acts as a bubble, anything within the Sigil borders is treated as on the material plane. Sigilians who die here during a crossing are dead, and demons who die here disappear, returning to the abyssal realm (even if it is only a few feet away).
However, the city has a unique interaction when its inhabitants step out of the borders. Any who step off of Sigil are treated as if the Abyssal plane is their natural plane so long as they remain outside of the city. Still, demons can be properly killed and their remains brought back to the city for harvest and resources collected in the same manner.
As the city crosses back, demons return to their home plane. Sigilians who have not returned in time remain in that realm, and are unable to return even via dismissal and other planar displacements.
Should a demon manage to remain in Sigil via unknown means, or is summoned into the city, they run the risk of creating what is known as a Demon Gate. Demon Gates are twisted structures that act as doorways into their respective realm. While open, a Gate will spread abyssal energy nearby (causing an accumulation of sin for unlucky residents). Additionally, they can serve as a gateway for demons to enter the city outside of a crossing.
The potential issue with Demon Gates is that while one is open, it tethers the city to that respective realm. Sometimes, Gates that are discovered that open to the Undersump are left alone, allowing the city to emerge in the safest realm for a few nights. It works the other way as well, Gates that open into more dangerous locations such as Charnelhome or the Barren Wood are best dealt with immediately.
Unfortunately, Demon Gates are exceptionally hard to destroy. It is possible to damage one to the point of no longer letting demons cross through and detaching from the realm, but it will still seep abyssal essence unless properly destroyed. To actually destroy a Gate, one must destroy both sides, meaning it will be necessary to destroy it from within the realm its connected. Thankfully, Demon Gates naturally decay after thirteen days.
Demon Gates take appearance based on their respective realm:
Undersump – A round portal made from rusted metal or waterlogged wood.
Ghahazi – A round or rectangular portal made from rubble and rock.
Midnight Isles – A round portal made from black vines and sand.
Cathedral Thelemic – A rectangular portal made from polished wood and carved.
Charnelhome – A rectangular portal made from polished wood and covered in vines.
Blood Clefts – A round or rectangular portal made from red stone, with blood stains.
Uligor – A round portal made from broken swords and rusted, cracked shields.
Pleroma – A round portal made from spiral carved polished stone.
Kurnugia – A round portal made from bone and horns.
Shibaxet – A round portal made from sticks and stone, similar to a nest.
Barren Wood – A rectangular portal made from knotted branches and covered in cobwebs and pine needles.
Ivory Labyrinth – A rectangular portal made from polished ivory.
Winding Wood – A rectangular portal made from dark wood and gears.