Not bad!
However, in the first couple of paragraphs the word 'sound'/'sounds' is used a bit repetitively. It can easily be replaced by 'noise' and 'racket' in a couple of places to vary the reading experience.
The monster is neat! But here is when the passive voice hinders the descriptions. Like the passive voice is fine to build up the anticipation with the slower pacing. But now that the monster is revealed, you want to be in the moment with your protagonist.
Like 'I feel his gaze upon me. A new form of terror invades me as I collapse' could be rewritten as 'His gaze pierces me, driving a new form of terror into my soul. I collapse." It's slightly punchier and more active.
Same with 'There are so many corpses here. It's as if they all died suffering. Their bodies twisted and warped into unnatural shapes as if a child has been playing with dolls.' The two 'as if' statements slow down the impact of the scene.
It could be 'Corpses surround me, twisted into unnatural shapes of anguish at the hands of some uncaring being with no regard for their toys."