I have a couple comments. Right off the bat, I'm amazed at how much detail you have worked into this illustration. It's really amazing, and quite impressive. You must have spent quite a while on this.
My biggest critique would be the lighting, or specifically, not doing it with layers. If your skull (which is rather freaky looking) is a separate layer, it becomes easy to create a layer on top of it, set the opacity of the new layer to very low, say 20% set the blend mode to soft light, and that way when you airbrush the lighting in, it will more merge with the surface of the skull, or the table, or whatever object you paint with it. Then, you can merge your light layer and your skull layer, and do it again a couple of times to really build up a sense of light and dark. I can link you to an example if you haven't gotten the gist of what I'm trying to say here.
My second, (much more minor critique) has to do with the way you have detailed things. You have done a great job in adding detail in all sorts of places, the flame, around the eyes, it's really great, but I think it would help the painting if in addition to line details, you also provided gradient details UNDER the lines, so the lines accent where the light and dark is falling. I don't know that I can say that a lot clearer than I am. You have some amazingly detailed line work, with not very detailed colors? I don't know. Mess with it, have fun.
You've done a good job here, I'd love to see more of your work! This certainly is creepy. Reminds me of when I read Faust. Not a pleasant read. Far too creepy for me. The guy sells his soul to a demon in exchange for magic. What's with that? That seems down right dumb. You know it has to end in tears. This painting provides that same sense that things are not about to end nicely, so I'd say you did a good job.
Early to bed early to rise makes a man stupid and blind in the eyes - Orson Scott Card