Font at scale is a little small for my liking, but you provide a printer friendly version which makes it a bit easier to read. As long as I don't have a pure garbage printer and hold it close to my face, I am probably going to be able to make out what is said. Old man eyes suck. The setup is clear enough. Gives a little backstory and direction. Feels like one I would use as a one-shot rather than try to integrate into most of my long-term games. That's fine.
As to the 'What's really going on' section, there's not a lot there not already implied by the first section. Mention of shadow creatures, but I would assume previous year's successes mean it's pretty easy to learn those are there. My bigger issue is it's another 'all or nothing' situation. Do the task or the terrible thing enters our world never to leave again. At least that's how it feels. As a one shot, that's fine. It does, however, reinforce the decision that this wouldn't be one I ran in a long-running game.
Environment stuff, sure. Not much for me to say there. Random encounters are varied and interesting. As long as there's enough room-specific encounters the lack of fights things is fine. Especially with the encounters being every round. To be honest, I would almost want a longer list of encounters with them happening so frequently.
Items are fine. Good enough information, clearly within the standards of the system. The crown isn't so powerful it's going to tempt a player to keep it at the expense of the world. Monsters include page references when needed. Shadow Curse is pretty brutal given the cure conditions and time frame of the setting. Finding the ISBN on a document like this is a bit of a surprise. My understanding has always been that those are mostly useful for placement in stores. Are you planning to try getting printed versions of this trifold into physical stores where scanning would be required?
Entrance memorial is a nice touch. Foreshadowing of the risk of death even in success. Room 2 is a dear god my players are F*ed moment. Not stealing, just moving them without reciting the prayer is enough to curse them. Granted, someone is stealing them regardless, but how many might move a few aside to check out the water? Moon reflection is nice though! I think I would make the number of figurines match the number of names on the previous memorial as a hint about needing to recite the prayer. I doubt most would realize, but in hindsight it would be a head-slap moment for them. The fissure offers options, though likely a split in the party.
Mineral vein is a solid treasure trap. Using up a full hour of the player's limited time for the sake of a payout. Value is enough to tempt a number of players I know, though the smarter ones would just note it on the map and try to get it on the way out. Room 4 is interesting. Kind of a bomb-shadow on the wall that can either be passed safely or turned into a foe depending on player actions. I like this. I always enjoy seeing situations where a player's own choices decide how events will play out. The only uncertainty is what I would consider to be 'inspects it too aggressively”. Probably meaning spending more than a single turn looking it over.
Room 5 is nice foreshadowing, but offers me no indication of what happens to the shadow zombies if someone gets rid of these bodies first. Most players loot and leave, but at least a couple over the years have been the sort to burn, bury, vaporize, or otherwise remove bodies from an area. Given the frequency of reanimation, I don't blame them. I wonder if burning the bodies or otherwise removing them would leave behind those shadows like found in room 4 to ensure an encounter later even if a different sort.
Room 6 offers a new exit option, a way to choose between suppression of a major problem or gain of value. It also creates a small haven and fall-back point for when players get into more than they can reasonably handle. I like this. Good design. Moving to the next room, the note about cave paintings overlaps the map. In the visual version, the white text disappears. Doesn't prevent me from understanding. It's a formatting issue though so something you may want to fix. This room also offers a second treasure trap. I might have said something here, but you offer a bypass of the time issue with a strength check. Doing so changes the nature of this treasure trap in a way to make it a unique situation. Ultimately, it is similar, but not the same.
There's an odd gap in palm sized silver medallion within the offering room. I half expected something here to have a shadow curse too given previous treasure tied to the location gave the curse if moved. Onward to the Ritual Room!
So I think it feels balanced. I suspect having a note somewhere at the start to ensure at least 1 player has to be a Priest, though honestly it is sort of implied. I also find it fascinating you note focus as a tier 2 spell instead of just saying focus DC 12. Neither is right or wrong, but by mentioning tier 2 spells, my brain did a hiccough. When I see the words 'tier 2 spell' my immediate reaction was to scroll back and check the character levels. Yep, 1st and 2nd level... I get that they aren't actually casting a spell, but by referencing the word spell, it feels off. It also eats more space, which given the space constraints... well, it's a choice. Like I said, not wrong. Hell, I don't even dislike it. I just think there's a mental disconnect it can cause for the reader. If you don't mind that moment existing for some people, no worries. If you want to avoid it, maybe just stick with a number.
So my real question at the end is this: What about the 'Shadowdark'? Is the Shadow Maw supposed to be the fail condition? The way the intro read, a great evil was going to spread into our world. Reading the 'if they don't finish in time' consequences, it's a Sarlac pit made of shadows fused into the floor of a cave. Those are two very different situations. Is the pit just the beginning and a final chance to undo the escape? Does the pit keep growing over time? I have a lot of unknowns here.
So as an overall, there's a solid little one-night event here. Depending on the group, it could be tied together well enough with an existing game. They can rush to the goal, they can explore, they can ruin their own knight with greed, and they can find multiple ways to handle the setting before them. At the end of the day, there aren't a lot of encounters until the goal, but for level 1/2 characters, that's probably just fine. I think it's good overall. Steeped in setting, power levels of items aren't insane, solid little map, etc. Having a 'save the world' game for low level characters feels a little odd, but given how it ends, I feel like whoever is asking them to do the job is an unreliable narrator. I picture whatever priest that was being overly dramatic. Like anticipating a category 10 hurricane and getting a foul wet fart instead. You don't want either of them directed at you, but the scale and scope is certainly not the same. Anyway, thanks for your efforts and glad you got this finished. It was a solid little adventure.