I think the transition between shading areas, both on the body and on the background, makes it look unfinished. Either add dithering, or an extra color or pair of between each shade.
The purple on the lower part of the background makes it appear closer to the eye than the rock (might have to do with the creature being dark, the rock light and the background dark again, plays games with depth perception).
The texture on the rock doesn't appear to allow support for the creature's feet. Maybe add some areas where it would be feasible for it to place its feet on.
Shading on the neck, lower torso and legs seems incongruent. I recommend trying to simplify the volumes, attempting to make them appear more cilindrical through shading.
These points addressed would make this disturbing although interesting piece better
