The Nazgûl had a sharp sense of smell. "The Lord of the Rings" states that their sight worked differently, too: "They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys; and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared." Their chief weapon was terror; it was so powerful that Sauron faced one disadvantage when using them: they could not easily travel in secret. Tolkien added, in "Unfinished Tales", that the terror they spread was greater when they were unclad and invisible; and when they were gathered together.