Great use of the title and I really enjoy how technical the text gets. Feels like a good way to give the DM some jargon to talk to the players when they interact with the visitors.
I agree with the others that mentioned that a section about the visitors and how they turn into diminished at the start would’ve been nice. Maybe you could compile the description of the decontamination rooms into the same entry to save space, since the text repeats.
Some minor additional notes:
- It’s not clear if the Predictive Armor and the Brutal abilities are already calculated into the stats, or if they should have an activation.
- In Shadowdark, looking for secret doors finds secret doors by default. Maybe what’s really missing is something that telegraphs their presence and prompt the players to search for them. If I was a player in this, I don’t think I would think about searching for a secret door in a decontamination chamber anyways.
- Same for the WIS check to identify Diminished as Visitors. Shadowdark doesn’t usually do knowledge checks, and I feel like you already do a good job to describe them so players can figure out what’s happening. From experience, it’s usually more rewarding letting the players guess at these things and then finding some information that proves them right and make them feel smart then just telling them the answer, especially since there’s absolutely no reason a character would’ve known this beforehand.