You know... you don't see it at first, only the unicorn. But then you wonder, what is rider looking back at? That's when you look at the large dark patch... and then the face. At that time I was like O_O what is it? I realised what it was after a second later. This picture was well done. So... well done.
Personal Opinion Time! I love that the dark doesn't seem "evil" like it often does in art. The dark just seems to represent the wild, unknown, and untamed. The rider of the white horse seems to support this aspect, in essence "taming" the light, making it safe and familiar. Incredible art, and instantly added to my favourites. Well done! <3