Its kind of hard to critique your work since its somewhat stylized
but on average the proportions of the face are a little off.
The face is too small for the skull and the forehead is too long.
With the values you are a little extreme and go too dark too quickly.
Try to be more consistent with contrast and remember that the background has less contrast
than the foreground
I wanted to say that I really enjoy your style, your pixel art in particular. Great colours and shading, I really love pixel art but never got anywhere near your level haha. Very refined stuff. Pretty cool subject matter too, it all seems connected. You know what you like.
"[...]as we gain facility of hand and travel further afield, we discover that we have a key to unlock the wonders of art and nature, a method of conjuring up forms at will: a sensitive language capable of recording and revealing impressions and beauties of form and structure hidden from the careless eye[...]"
-Walter Crane, 'Line & Form'