That is amazing, I really love the highlights you have put though the eyes/ iris, the shading you've done through the skull I really like to and very detailed. Now I really think this peace is amazing but two things I found off I think it could off done with some more highlights on the whites of eyes and the background is a little flat now obviously you dont want to take away from the focal points and it is a very fine line I understand this all to well, but I still think maybe some shading around the big eye (or something else) to bring it out more or something would of just give it that bit more of an edge.
Either way your this artwork is stunning and I think you should be really proud of it.