Morgûl-mask

Many of the Dark Lords servants wore helmets with horrible masks, best known of them the Mask of Morgûl worn by the Witch-king of Angmar.These kind of masked helmets maybe were inspired by the original dwarven masks and later adopted by the dark lord´s mannish and orcish servants.

speculations
As the Witch-king´s mask is hardly described it is difficult to say what Tolkien had in mind.Historical masked helmets include the Imperial Gallic Masked Helmet, the viking helmet, the Persian Masked Helmet or the samurai Helmet.