In the beginning, all that is known about Silver Mask is his obsessive desire to torture and kill both Andragoras and Arslan. He is serving as the leader of the Lusitanian forces but is in fact a Parsian himself and soon commands a regiment of collaborators. He is so named because of the ornate silver helm he wears; it has large silver bull-horns attached to it, and completely obscures his face.

He later reveals his face to Andragoras, now his prisoner, who recognizes him as Hermes, the son of King Osroes, Andragoras' brother and predecessor. Hermes' face is partly burnt by a fire he accuses Andragoras of setting as part of the coup that made him king.
(Source: Wikipedia)