OMG, this is awesome! The anatomy is brilliant.

I think your idea is great with all the darkness. Perhaps keep Sam as the brightest part of the picture? Or imagine this: black as background, then as anniespinkhouse said, something fiery as lightsource which is then partially blocked out by wings. You could draw most of Sam in the shadow (from darkness and the shadow of the wings) with sharp contrasts of light where he catches the light of the fire?

I truly hoped that made some sense.It completely does in my head ;)
Either way, this promises to become a great picture :)