This is so good! Everything you mentioned about tonality is on full display and works really well to create a sinister aura.
The sections being phases works really well and I can definitely imagine this in a souls-like boss fight that has multiple phases.
The orchestration is fantastic and reminds me of the folk horror of Midsommar (especially the cluster voices).
The contrast between sections A and B is really nice and allows the listener to ponder the lore behind the boss (very souls-like).
I did like the overall dynamic curve of the track but I would have liked to hear a bit more of a ramp up in intensity towards the end with maybe some faster lines underneath the main melody.
Overall amazing job you absolutely aced this style!