The image is very, very dark right now. I know it’s underground, but you need to think about getting some more light in there so that people can actually tell what they’re looking at. Maybe you can throw in some other light sources - glowing crystals/moss, lamps. I’m having a hard time actually telling what the creature’s design is at the moment, mainly because it is so dark, but I’m not convinced by those horns or how very reflective his skin is. His horns look like they’d get him trapped underground. If you’re designing an underground creature, you should be thinking about what features they use for digging, how they navigate in the dark (is it very good sight, sound, smell, infrared?) and what they eat. Underground, there’s not much large-scale food, so why is his mouth so big and full of tiny sharp teeth?
Compositionally, I’m not fond of the large dark splotch running straight up in the foreground on the left, and the rock formations pointing downwards in the background are really uniform, also pointing straight down, which giving you a lot of vertical lines on the canvas, and forming tangents with the top of the guy’s head.