Awesome stuff, especially with such a broad prompt! The cover is great and paired with the following hook and explanation, really set the tone. The implied setting here is right at home in Appendix N. and the magic and creatures present in the castle really sells it.
The Axe, Mercy is a fun magical item and potentially very useful too, it is tied in well to the central tension of the module. Each room key is clear and well formatted and the layout is clean and well-presented, the only error I can see is that the room keys are labelled with numbers, but the map itself shows the keys without numbers (not something that makes it unusable, just something to note, in case you wanted to change that).
Awesome work and I'll definitely be running this!