The Fall of Barad-dûr in The Return of The King
“But the Nazgul turned and fled, and vanished into Mordor’s shadows, hearing a sudden terrible call out of the Dark Tower; and even at that moment all the hosts of Mordor trembled, doubt clutched their hearts, their laughter failed, their hands shook and their limbs were loosed. The Power that drove them on and filled them with hate and fury was wavering, its will was removed from them; and now looking in the eyes of their enemies they saw a deadly light and were afraid.” — J.R.R. Tolkien