I love the busier sections. You effectively established a dark and sinister mood, and I feel like the composition says a lot about the character.
I found the repetitive and prominent organ line difficult to listen to for a long period of time.
I think it would be nice if there were some interspersed unexpected major chords, which I found can be quite useful in enhancing a dark atmosphere.
I wish there was a section that provided some contrast from the rest of the piece. Perhaps you could have made a lighter section that has the same mood but with lighter instrumentation, such as flutes, celesta, glockenspiel, etc.
I enjoyed this a lot despite these minor gripes :)