Well, let's start with general look. He looks menacing, demonic and very powerful. The cape helps with that by giving him a presence of authority, like a king, further emphasized by the crown-like horns on his head.
His colors are a very consistent black base with accents and highlights of purple and silver.
It looks very posable and pretty well balanced, the tail seeming to be helpful in extra poses for stability and effect.
The head, while using a lot of technic and system stuff, definitely pulls off what it's going for: a mighty demon of darkness.
The proportions look absolutely fantastic and natural, not drop-dead perfect 1:1 to a human but really flipping awesome nonetheless.
If I were to change one thing, it would be to see how he looks with black Vorox shoulder piece on the top of his face to round it out a bit more but that would require making a new head design.
Aside from the flat head, I honestly have no complaints. 8/10 from me because there's always room for improvement, that's one of the fun things about Lego.