This write up is designed for Game Masters who want to flesh out a detailed idea- hence it lacks a list of monsters, or rooms, or treasure. That’s your job.
The rain here smells like copper and decay.
Nothing moves in the swamp, where the everpresent damp might as well be a permanent curtain of water, be it through torrential downpour or the chill tendrils of mist that seek to ensnare the unwary in a blanket of confusing, frigid death. The swamp leaches and spreads for leagues in every direction, and the church that suddenly appears from the mist stands out from its decaying clutches like fire on a wound. Nobody quite knows when it was built, only that it is old- once manned by a secretive order of monks who aided travelers through the marsh before retiring to their chapel to pray and study. But those quiet helpers are long since gone, the memory of their purpose gone with them.
Now, nobody stays in the church, at least not for long. Those who look upon it are afflicted with a sense of overwhelming grief by the sight of its broken parapets, once proud walls now crumbling with time and age. Anyone who enters the church soon leaves, driven back by a feeling of intense discomfort. The only reliable constant in all eye-witness accounts is the larger-than-life statue of Saint Ithiel that dominates the entrance way, complete with a pair of enormous bronze wings. Ithiel Waxwing, who dared to touch the sky. Ithiel the Helper. Ithiel, the Ashfeather Hex.
Every once in a while, the swamp is wracked by a muffled thunder that sounds through the water in a curious double clap. Some claim that they hear a soft laughter amongst the echoes, but such tales are usually stifled unless behind locked, barred doors.
The rain here smells like fear.
First off, Saint Ithiels Solace is a church, not a dungeon. That being said, it should not have barricades, or an armory, or a defensible structure of any kind (unless a valid reason can be supplied). Astute characters might realize the worth of a library in this manner of monastery as a collection of knowledge- others might recognize that if the monks were the only ones who truly knew the swamp, they would also be the only ones who would collect treasure from the dead, not to mention whatever secrets the swamp held before even their time.
At any rate, half of the church is probably submerged due to the fact that it is a heavy structure built on a wetland- unless a suitable reason can be procured (such as the entire building is waterproofed, or magical pumps hold back the tide). Thus, the adventurers are going to have to learn how to swim. Because it is so isolated, they will also have to worry about hypothermia from the freezing and wet conditions, but should also be wary because any fire will attract the attention of the creatures that lurk within the swamp, as well as the deeper darkness within the church.
While you can fill Saint Ithiels Solace with whatever manner of dark gribblies you feel like (a druid obsessed with moss and lichen would do well in here, or perhaps the last, insane monk and his cohorts) the plot remains rescuing Saint Ithiel. If this happens, an angelic figure with feathers made of wax appears in a dream or vision as crying for help- any talk with the locals will reveal this to be Saint Ithiel, and they will tell the characters how to get to the church. On the way to Ithiels Solace, the characters will feel the double clap of thunder, and possibly a soft laughter.
Upon entering the church, they find it mostly deserted except for monsters that are somewhat lacking of higher thoughts- enormous rats, constructs of muck and rot, perhaps a few zombies. Anything that lives in a marsh and is mostly brainless can be used at this point.
The deepest level of the church, and the most submerged, should be the bell tower and the room directly below it, sunk deep into the ground by some forgotten calamity, and only accessible through a natural underground passage. In the room is a faintly angelic figure inside a glass-like wall of force, who identifies himself as Saint Ithiel, and claims that he has been imprisoned for half a millennia by the order of monks, who drew power from his agony. If the characters attempt to free him, he says that the only way to break the wall is a willing sacrifice of pain, which Ithiel can direct to break the walls that hold him. He warns that it will be excruciating, and if a character submits then Ithiel kills them. If the characters are surprised (and they probably should be), Ithiel reveals himself to be a demon, and attacks the rest of the party.
The order of monks originally imprisoned the demon, and attempted to garner knowledge from his infernal repertoire of thoughts. Unfortunately, this caused every monk to slowly lose his sanity, until finally the order died out. The demon is trapped in the tower, but manages to sustain himself on the fear caused by the swamp and also direct consumption of people trying to rescue him. He tolls the bell whenever he kills, which is what causes the double clap of muffled thunder.
Ithiels original tomb is still in the church, and may be found before or after the confrontation with the fake. Obvious treasures include the saints equipment (though stealing from a religious figure is somewhat of a bad decision), whatever treasure the demon accumulated, as well as knowledge from the library the monks stored.