I think the concept is good. I did struggle with the gameplay overall, not because the game itself was difficult, but because it was hard to navigate. I played it a few times and other than the first key / door. Whenever I found a key I couldn't find the corresponding door. To echo, catdotjs below I think a minimap would have helped with that.
Other areas that I think would benefit from some additional time would be the audio, I do think the general hum in the background works as a non intrusive base audio, but I think that is the only audio in the game. I think some audio on attacking, absorbing blood or picking up items would be beneficial.
Theme wise, the game very clearly hits the mark and I think the core game loop has potential if it is fleshed out a bit more.