Commentators have written that the Nazgûl serve on the ordinary level of story as dangerous opponents of the Company of the Ring; at the romantic level as the enemies of the heroic protagonists; and finally at the mythic level. Tolkien knew the Lacnunga, the Old English book of spells; it may have suggested multiple features of the Nazgûl, the Witch-King, and the Morgul-knife.
Commentators have written that the Nazgûl serve on the ordinary level of story as dangerous opponents of the Company of the Ring; at the romantic level as the enemies of the heroic protagonists; and finally at the mythic level Tolkien knew the Lacnunga the Old English book of spells; it may have suggested multiple features of the Nazgûl the Witch-King and the Morgul-knife