I really like the Victorian setting here, which I think fits with your graphic design perfectly. You get across everything you need to need to run while making it evocative!
I’ll weigh in on the letter discourse, haha. I think that all you really need is an excuse for why the letter is cut off, and I think the envelope fails to accomplish this. The letter was addressed to Magnolia and presumably instructed her on how to live again; it makes sense that it never reached her (indeed, I think it fits perfectly with the debt-collector position of the party and the letter’s own warnings!). I think that essential premise is wonderful, and ripe for party conflict, too, so from a game-design perspective full marks. However, if Flamel wanted his bride to achieve her immortality, and include instructions to that effect in the letter, you need an excuse for why the party doesn’t start knowing those secrets, precisely because they seem so important to how the party approaches the dungeon. Ripping, burning, or (my recommendation) chemical stains would accomplish this in a pinch, but I wonder if something that seems less like a “narratively convenient coincidence” could be done instead… not sure.
I want to finish up by saying that I really, really like the way you use the souls as they pertain to the animals. It’s such a clever move to make a chimp bark and act like a dog, because it communicates the premise of soul-swapping, horror and humor all in the same stroke. That is design accomplishing more than the sum of its parts, which is the mark of great design.
Wonderful work, make more! <3