When thinking of what to add, you really need to also consider what to take away.
First set your intention. I assume yours was something like, "I need a dragon, and a knight, and I need to have him eerily coming out of the cave." And if its for concept art or illustration. What story are you trying to tell. If its environment and scale, then shiv0r's paintover does great.
However, if you your intention is, "I need to scare my audience, and show a good dragon design"
Your focal points are the dragon and the knight. All the extra stuff in the composition you need to consider if its necessary. Consider a crop in.
Your brightest light values and darkest dark values need to be your main focal point. Lead the viewers eye around the canvas to your focal points. You have a light knight on a light background. Which makes for no contrast. Make him a light knight on a dark background. Your cave, is an element you have to play with however you want.
For illustration follow a simple rule of dark on light, and light on dark for now, and frame your piece based on your focal points and storytelling elements of composition.
Cheers.