The only time in my life that I've seen something like this was in a middle aged woman. She was possessed by an evil spirit and needed an exorcism.

When her voice changed into a deep, malicous and thoroughly evil voice that cannot be properly described because there are no words to describe it, her normal eyes changed completely black. No whites or pupils. Nothing but black darkness.

It scared the shit out of me. Hope this helps you understand a little more.