Also i decided to go with deep blacks for several reasons. First it helps maximizes the contrasts and feel confident about the lighting, but the main reason is that i wanted a treatment that fits the genre of this illustration (fantastic). I thought that having large dark areas in which the details are lost helps to bring more ambiguity. You can't see clearly the monster. It's almost only defined by its silhouette and a few intriguiging elements and this is almost what the fantastic genre is about, not being sure of what we see, questionning the perception.
Light always plays an important part in classical fantastic movies/stories. Just think of Halloween when Micheal comes slowly out of the shadow or the first Alien, whose design almost consists of being a dark spot (idea brilliantly re-used in Attack of the Block, which i always recommend to all my friends to see).
I think a good monster design is not always about an accumulation of weird details but is also about what we don't see, or even better what we can't see, which makes the viewer wonder. So the way a creature will be lit should be thought of during the design phase.