im stuck... but this needs work... thoughts?
Cool character! He's getting lost against the background a bit. When you work monochrome, you've got to really separate the silhouettes with a strong value/tone plan. I'd experiment with either lightening the character or darkening the background or both. It will probably help him to stand out better.
There's too much texture in my opinion. Everything is textured and everything kinda looks alike. Try to paint the objects, then you can add texture on afterward and you may not need it in the end.
The rim lights you're using is looking good. Rim lighting has got to be the foremost trick in concept art! Makes anything look better
I'd lose the plain black in favor of a dark violet. It's generally frowned upon to use black in art because nearly everything has some subtle hues in the shadows. Also, try squinting or blurring your eyes while looking at the painting from a few feet away. Right now, you can see the ribcage very clearly, but the head and the limbs blend with the background. Also, I'd suggest adding some more variation in the hues of the whole painting. Right now, you have the cyan and the pure blue and not much in between. In professional concept art, they use a ton of subtly varying hues even if they're going for a piece that's mainly based around one color. To do this, you could try making each color from scratch instead of color-picking the colors that are already on your canvas.
You have some great stuff going on here. I like the creature's gesture and the overall dynamism of the piece. As rseward mentioned, there are some problems with your figure/ground relationships and also the overall arrangement of your values. There is also an issue (composition) with his sword being so bright, and cut off the way it is. It ends up leading us right out of the image.
I think if you make the background a bit lighter and slightly more subtle, and save the darkest darks for the figure, it would help define him more. Also, establishing the direction of the primary light-source more clearly will help define his forms. I would also suggest that you use some warm neutrals in the image to help accent the cold light of the moon and environment (it also adds nice variety). I made a small painting to show you some ideas. I extended the image slightly to include the full length of the sword. I also tried to keep the lightest lights and darkest darks in the area around his face, and made the glowing sword less contrasting, to create a clear focal point. Hope this helps and good luck with the finish.