Lord of the Nazgûl

The Lord of the Nazgûl was the title of the Witch-king of Angmar. It was also often used to refer to the Witch-king himself and indeed is used more by J.R.R. Tolkien in the text of The Lord of the Rings than 'Witch-king', which appears only in the Appendices. After the Witch-king's demise, his second-in-command, Khamûl, would take his place.