This is looking really nice - glad you were able to recover some files! (I lost one file during this challenge and so I ended up taking the JPG it’s saved of it to post here, blowing it up, and painting over it!)
I don’t think you need to go any darker: Mood-wise, it’s really clear that this is a night-time scene and overall what the tone of the movie is. I do like your idea of adding body parts to the water, and maybe a hint of red down there in spots too…?
A couple things seem a little odd to me: Of the 2 secondary characters - the one gnawing on the leg and the other with the axe - both seem about the same distance back from the camera… but one is in the muck up to his upper ankles and the other is almost up to his knees. Maybe too late to fix, and it may not even matter - no one said the swamp floor had to be even - it just caught my eye.
Second silly nitpick: It seems funny how the source of the lightning, where it hits the clouds, is obscured by a tree branch. Valid, just seems like something you’d show clearly, dunno why.
But overall, aside from my picky comments, this is looking terrific! Very appropriate to the theme. Nice work! 