As a heads-up, I'm usually super critical of technic/constraction MOCs, but, uh, whoa. This is one of the best that I've ever seen.
From head to toe, this guy is well defined, sleek, and muscular. He or she has a legitimately lifelike appearance, which is pretty tough to nail down. The color dichotomy that's capitalized on with a decent but not-too-liberal number of translucent pieces is awesome to look at, and the fire/ice breath is a clever touch.
The only serious critiques that I can think of are that the unique pattern of the Darth Vader chest plate clashes slightly with the area around it, and that the spikes near the top of the shins jut out in a pretty unnecessary and disruptive way. Again, though, this is incredible. Congrats, man.