If you just have one light source, have a look at the ghostshooter tutorial, that explains how to do such an effect without using too much gpu load.
If you will have several light sources with different behaviours, movements, rotations, animations, colors and the like, then Color Fusion (Masked) fits perfect.
Alright, but just so we're clear on the effect I want to get, "I like to be sure". Objects outside of light will not just be darker, but a whole different color. Silhouette. The reason this is so important, is because a creture in the dark could have a darker body, but white eyes that show clearly. I probably should have mentioned that earlier. :P
Either way thanks for being so helpful.