A suggestion for making it dark without being too dark is to render it using 'night' lights at a slightly brighter setting and then adding 'gloom' via postwork. A black to white fade used to highlight the main figures or dramapoint of your image can be overlaid or multiplied in a secondary layer. I often use a slightly blurred layer of the image itself overlaid and then partially erased over the main area in addition to the fade(s).