The Nazgûl were once Men, until Sauron gave them nine Rings of Power. These proved to be their undoing, they eventually became invisible to all save he who wore the One Ring. Only through Black cloaks and hauberks of silver mail were they given form. Though their human form was completely gone and invisible to mortal eyes, except through their attire, their hypnotic red eyes, which in a rage appeared in a hellish fire, could still be seen.