Helm of the Ringwraith of Forod

Though the Wraiths had previously been portrayed as pale kings in 'Ring vision,' the film's director wanted them to assume new forms, as if spiritually armoured for battle.

Summoned from beyond the grave by their master’s will, the Ringwraiths rose to challenge the White Council in the tangled ruins of Dol Guldur. Spectral, formless beings, the nine Wraiths could not be seen by mortal eyes lest they robed themselves to walk among the living. The Elves and Istari of the White Council perceived beyond the limits of mortal sight, and to them the Nazgûl appeared as warriors, clad for battle in black mail and plate.