Our story begins in Shardis, a large port town along the banks of the Malvarin River. Unbeknownst to almost anyone the city lies along a supernatural fault-line, a seam in the fabric of the planes where the potential exists for denizens of the Hells to slip out of their prisons and roam free once more. To this end a small cult known as the Children of Midnight has prepared a ritual of dark magic to open a portal to the home of their masters, the Hell of Primal Shadows. The ritual is centered around an eclipse, long foretold and known to be occurring on a certain date. This eclipse will shroud the entirety of Shardis in darkness, and at the moment of totality the portal shall be opened.
However, this is not the culmination of the story, only the prelude.
Stage 0: Pre-Eclipse
Lasts until the Eclipse
Before the Eclipse occurs, Shardis is a normal port-city. The party can interact with everyone as they would in any other location. No monsters lurk in the streets save for the usual, human kind. The city guard is bored, the shipping trade is thriving, and the populace as a whole is content. Visitors are encouraged to visit the awe-inspiring Shining Towers, and scattered throughout the town (more heavily near the docks) are as many sources of entertainment as you could want in a big city.
As the day of the Eclipse draws near the city begins to whisper and murmur, for an eclipse is always an interesting site to see. There is almost no fear however, because the priests at the Temple of Lights have made it a priority to assure the residents that this is purely a natural phenomenon.
The most attentive of listeners will hear an interesting rumor: there have been disappearances around the city. Nobody important, but here and there a house has been found completely empty, the family missing from their beds. Transients and the homeless beggars have been hit the hardest, with only a fraction of them remaining by the morning of the Eclipse.
If the party is out and about late in the night, they might be ambushed by a small group of men in dark cloaks, one of a dozen such collections of cultists who have been tasked with gathering sacrifices for the upcoming ritual. Thus far they have gone unnoticed by the authorities, thanks to careful planning and targeting of victims. No blood has been spilled, no evidence of wrongdoing has been left, just people gone missing.
The kidnappers will attack using saps, nets and if possible, trip-lines. If the initial ambush doesn't give them a reasonable chance of success, they will retreat quickly and quietly. Above all else, the cultists will try to avoid being captured, because that risks the secrecy of the ritual.
In a similar vein, members of the party might be propositioned by fairly attractive ladies "looking for a night of adventure". If the PCs take them up on their offer, they'll enjoy a wild night of passion, but the female cultists will do their best to drug their marks so that their male compatriots can carry them off in the dead of night.
The day of the Eclipse, the Temple Market will slowly fill up as the morning passes. By the time the Eclipse starts at noon the entire area will be even more full than normal.
GM Note: Mystic Eclipse
The Children of Midnight have been kidnapping people as quietly as they can for the week leading up to the Eclipse, to be used as sacrifices in their dark ritual. Once the Gates of Hell are opened the all-consuming darkness therein will bind the Eclipse over Shardis to itself, strengthening it and blanketing the city in a mystic darkness that allows the demons to roam freely.
This Mystic Eclipse will last until the portal is closed.
Stage 1: Evening Twilight
Lasts for 3-5 days after the Eclipse, until the sun no longer shines
The first sign that anything is wrong comes at the moment of totality, when the starburst motif on the front of the central spire of the Temple of Lights begins to glow with a steady light. The Eclipse fades and the sky begins to brighten up again but it does not completely return to full-noon. The sun burns in the sky with a muted luminescence that grows dimmer and dimmer as the days pass, until one morning the sun doesn't rise at all.
GM Note: Monsters in the Dark
With each stage of the Mystic Eclipse the portal to the Hell of Primal Shadows grows in strength, until Stage 3 when it is completely open. While it is open the inhabitants of that dread plane can make their way into the city, but the more powerful demons will not be able to pass through the gate until the later stages.
The gathered crowd will murmur and talk but none of them have seen an eclipse before, so they aren't entirely sure if the muted sunlight is an aftereffect or not. The reassurances of the priests beforehand have given everyone a sense of, "It'll be alright, it's just a natural phenomenon, the sun will be back tomorrow," which prevents a riot from breaking out. The crowd disperses without incident, the people going back to their daily routines.
On the second morning everyone will be a bit shocked at the muted light of the sun and rumors start to circulate that something is wrong, but mass panic hasn't yet tightened its grip on the populace. On day three however, the riots begin. The first few are small and easily put down by the city guard, but around mid-afternoon the city is in a state of panic and a mass riot starts in the Temple Market, blaming all visitors and outsiders for the dimming of the sun.
A great many people, no small number of them residents of Shardis, are lynched by the rampaging mobs, their corpses hung from the lampposts or nailed to the walls of buildings. Sane people lock their doors and avoid going outside, or begin leaving entirely. The city has gone mad.
Encounter: Lynch Mob
After the third day, every moment the party spends in public increases the chance that they'll become the target of a lynch mob. Only quick wits or quick feet will save them from an unthinking horde of rapacious looters.
Stories are circulating, hushed whispers of things haunting the darkest streets at night, stalking those who travel them. Lamps and torches are found destroyed every morning. A true atmosphere of fear sets in, and it's all the city government can do to keep their own people working.
Amidst the general chaos of the rioting more kidnappings occur, and the cultists become bolder about it. On the last two days there will be groups of cultists masquerading as rioters, attacking and kidnapping entire groups of people at once for fuel to strengthen the portal and the accompanying mystic eclipse.
Stage 2: Darkness Rising
Lasts for 5-7 days after Stage 1 ends, until the quarantine is enacted
By midnight of the fifth day the portal has opened enough to sustain the eclipse by itself, and the trickle of low-grade demons sneaking into the city becomes a stream of horror. The sun has been banished from the sky and the invasion has truly begun. Supernatural darkness begins to seep throughout the city, dampening the illumination provided by all lights.
The morning when no sun rises is the point at which people really start to leave the city, though most decide that they can survive one more day while packing their valuables and preparing for an extended time away from home. Unfortunately for them, this is the point where things truly begin to devolve.
The priests of the Luminescent One and the city guard are quickly inundated with reports of nightmares stalking the streets and entire households slaughtered. The priests begin to organize sanctuary and relief efforts with the local guard posts, but requests for orders from the Mayor and bureaucracy are met only with silence. With a lack of centralized organization little active help is available to the populace at large, though the guard posts become beacons of sanctuary, guaranteed to be lit and (relatively) safe in an increasingly-hazardous city.
In a bold move on the second lightless day, the cultists are able to sneak into the control room within one of the three Shining Towers and sabotage the spellsphere that powers its magic. This spellsphere is then subverted by a shadowbeast, which becomes more terrible than any of its brethren. The newborn Swollen Shadow begins to haunt the trampled and ruined remains of Shantytown, making it almost impossible for citizens to flee the city. The noose has been set and is now slowly tightening around the citizens of Shardis; with no avenue of escape, they're sitting ducks for the monsters roaming the streets.
The severity of the situation is punctuated by a demon attack on the City Hall that removes almost all of the bureaucrats who refuse to recognize that anything is wrong; this fortunate turn of events clears the way for Guard Captain Aryna to take emergency control of the situation.
The priesthood and the emergency government come to a mutual conclusion: the city must be quarantined to prevent any of the creatures from escaping to wreak havoc on the countryside, as well as prevent the further spread of the lightless sky which has been slowly expanding to the area around the city. To that end priest and mage combine their talents to forge a mystic barrier that encircles the entire city, allowing people to escape but trapping the fel creatures of midnight and their unearthly darkness within.
Stage 3: Eternal Night
Lasts until the portal is closed
By the time the quarantine is enacted the population is a third of its original size; Shardis has quickly become a lightless ghost-town. The city guard and priesthood make daily incursions into the city to rescue survivors and attempt to gather information, but after a few days their efforts begin to lag; a dozen forays each day becomes a handful, which quickly dwindles down to a single attempt per day.
The survivors are scattered across the city, though there are a few larger clusters. One cluster sits in each of the remaining Shining Towers, and a third cluster in one of the guard posts. Within a day or two of the quarantine, the guard post cluster is overrun by the sheer number of invading demons.
The second Shining Tower is brought down a few days later, after a siege that ends with the use of an unholy relic: the skull of a Priest Hunter. With this sacred artifact the Children of Midnight turned the light of the Tower against its defenders. Now the building stands as a testament to the power of darkness, its once-bright halls now filled only with silence and the stench of blood.
Once the second Tower falls, the only large gathering of people left is in the sole remaining Shining Tower. After approximately a week, it too is overrun and the occupation of the city is effectively complete. Demons roam the streets and buildings, hunting down the last remaining survivors. The last sanctuary is the Temple of Lights, which will remain under the aegis of divine protection until the eclipse ends (if ever).
At this point the Quarantine itself will come under attack. How long it can last against a full demonic invasion is anyone's guess, though the Guard Captain will eventually determine what is going on and lead a final incursion into the depths of the city to close the portal. Success is a long shot, but it's the only option left to those who once lived therein.
Stage 4: The Light of Dawn
Begins after the portal is closed
When (if) the portal is closed the unnatural eclipse will dissipate: without the hell-gate to support it the mystic working begins to unravel. Such a creation doesn't fall apart immediately however. Over the course of approximately a week the eclipse will fade and the cleansing rays of the sun will again reach the city streets.
Most of the demons will either flee back through the gateway or be utterly purged by the sunlight, but no small number will instead seek refuge in the lightless interiors of buildings all around the city. Finding and clearing them out will be the work of many weeks. The city sewers are particularly bad, and will likely be a source of horror stories for years to come.
Once the city is mostly cleared of demons the populace will return and begin the process of rebuilding, but the city will never be the same.
GM Notes: Though this chronicles the events of the Eclipse from their beginning, that does not mean that it must be used as such. The players can stumble across the city right after it has been quarantined, they can be sent in on an against-all-hopes rescue mission after the city has been utterly consumed by the invasion, or any number of other hooks. This simply gives the GM more options for utilizing the Eclipse and the invasion; in-fact, the PCs might simply abandon the area right at the start and later hear about how things have really taken a turn for the worse.
An alternative option unexplored here is the idea of using the quarantined city as a recurring location for the PCs to visit. If the portal is never closed and the quarantine lifted, then the city would remain a dangerous location with almost limitless potential for reuse. If this is the case, then the "final incursion" by Guard Captain Arnya in Stage 3 will have failed. "The cursed city" might even become a widely-known location throughout the lands.