Wow this is amazing. It is honestly one of my favorite pieces of art ever. The colors that you chose were perfect and I think that the blue matches very nicely with the background. The lighting really helps emphasize the center of the picture and the level of detail is amazing.
I really like the way that you made the water and although it isn't the most realistic water that I have seen painted it still looks great. The lighting is great and it really draws your eye to the center of the piece. It also helps that you chose darker colors for the background. I think the very subtle red at the bottom of his pants look great and I like the way the tentacle looks sort of shiny. The mans hair also looks very good.
You have a great level of detail on both the main part of the picture and even the background looks detailed with the gold lines and the way the wallpaper looks. The octopus is colored very well and looks realistic. I also like the mask the man is wearing it looks very detailed and I love the color of the mask. The guy's pose was also good although I think it could be improved a little more. The tentacles have a good pose though.
The mood of this is also very cool. It has a very mysterious feel because of the blue and the mans mask. It is interesting to think of what is going on in the picture and I think the way you have the lighting adds to the mysterious feel even more. I really love the expression on the mask too, it almost looks sort of creepy. I think that you are very good at portraying this feeling in your drawing and it looks very nice because of this.
One of the thing I think you could improve on is the detail in the clothing. It looks nice, but it would look amazing with just a little more color. I also think you could improve the detail on the water and maybe you should add some water dripping off of the tentacles too. Other than that it is a great piece of art. I love it. (:
You are a really great artist and I love your work. I can't wait to see what else you can do.I will review you other art later too, just not right now.