I found him really well done, and the fact that it would never exist as a set was a huge disapointement. Such vilain would have been perfect for the third year if the story took place in the shadow realm.
So I tried to recreate him in stud.io, and here is the result:
Demon Warrior is one of the best concept art designs, this is basically a perfect recreation of the art. However i do feel that, were this a real set, the weapons would have more of that weird crystal detail that the later G2 sets had.
Looks pretty accurate! I really like the color scheme, the bright neon green and purple combined with the darker black is quite cool looking. The weapons are good too, overall a very nice moc
I know, because I’m the maker of these pieces. At the begining, I didn’t planned to do a mask, because it wasn’t clear if the character had a custom head or a mask, but due to your suggestion on thingiverse, I started creating something. I don’t know how long it will take to finish it, because I’m more a weapon modeller than a mask one, but I’ll try my best.
That mask is pretty accurate to the artwork as well; I personally would have just stayed with the bare skull, but this definitely adds to the character and is probably more realistic to what Lego would have done if they actually made this character into a set.
Looks cool Might be having a go at this character my self with your parts too Did you consider what I said with the shield at all? It looks like it should be a bit curved and a bit of detail or texture would be really nice too.
Not essential but just my own opinion
Might be having a go at this character my self with your parts too 
Have you added it to the pack yet? Cus I’d love to start this right now haha