You did a great job immediately catching the reader's attention and not overwhelming them with too much information. The whole first page is especially fantastic. It would be up to the GM's interpretation anyway, but I'd suggest cutting "but none are really evil" from the section Before the raid... - maybe some of them actually are evil.
I love the tables to decide what players gained, lost, and would suffer breaking from their Oaths.
The adventure feels very grounded due to its historical references, but at the same time mysterious and dangerous with the references to the occult - especially as it proceeds. Fantastic work on the layout and the illustrations.
To maximize usability, it might be worth considering swapping the names of real places/countries with imaginary ones. Of course, anyone using the booklet can do that themselves but it would save them some work. It would still be a nice touch to include references to the historical facts that inspired the adventure, but maybe just at the beginning and end of the booklet. I also recognize other readers might disagree and prefer the current setting.
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Thank you for your in-depth review, really appreciate it ☺️ I completely understand your suggestion of using fantasy names and places to make it a bit more useable in any another OSR campaign. It's not easy to drop the buckriders in somewhere. But I did wrote it with the intent of a self-contained one-shot (multi-shot maybe) and the historical setting is a feature of that. Ofcourse any GM may turn things upside down to make it fit their story. I would encourage! 😉
Btw, being a historian by profession it's a bit of a trademark to use historical themes in my games with my regular game group ☺️ haha.