It is mentioned in the chapter 'Siege of Gondor' in The Return of the King.
From the book;
The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter. "Old fool!" he said. "Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!" And with that he lifted high his sword and flames ran down the blade.
He only wields it once as shown in the books. He swapped it for a giant mace during his battle with Eowyn.
He bent over her like a cloud, and his eyes glittered; he raised his mace to kill.