I will be very nitpicky now. Don't take it personal, it's the loving slipper so you can improve.
- The background building is way too dark, it not only takes focus away from the figure in the middle, it's also unnatural that it's not taking any color and light of the surrounding flames.
-
View attachment 48380 the lines are touching, which makes the head and walking terminator look close instead of being far apart
- the block of green on the right is unclear, I can not tell what object it is nor how far it is supposed to be
- the flames could have more hue/value diversity, if you analyze the original it has mainly orange, red and green that are far apart in hue, the ref artist brought them together by varying the saturation and altering the value with it, thus making them meet in the grayish spectrum
- overall your composition could have benefitted from more midground
- you got foreground with the big head on the left, background with the burning castle, but the midground is practically one with the foreground
- the horizon line is extremely low; the ref got away with it because a)the man and small severed heads are close to each other b) the big heads are overlapping/cover part of the small heads, indicating that they are in front c) the heads on the ground are actually not that extremely different in size
- yours has two heads in extreme size gap to indicate distance, but without overlap, the waterlines that touch each other make the sense of distance is ambiguous