Saint Sebastian Attended by Saint Irene

I find this to be an amazing piece of work for several reasons. First, the technical dexterity is world-class. Second, the composition is outstanding. There are basically three visual stories. First, the lightest and most obvious figure of Saint Sebastian in the foreground. Second, the two attendants. Third, the space in the background that the top attendant looks upon. What is that space back there? Is it a cave? Is it the door to hell? It's definitely dark and foreboding, so I want to know. 

This was probably a study for a painting, but I doubt this would work as well painted, or in color. With only charcoal, chalk, and paper, the artist has constructed a whole world that draws us and gives us a sense of place. I think it is amazing when an artist can do so much with such basic media.