From Tatters of the King
The Pallid Mask is alluded to, but never described by Robert W Chambers in his stories of the King in Yellow.
In the play Carcosa, or The Queen and the Stranger, The Stranger wears a featureless white mask of which he tells the people of Yhtill to wear copies to protect themselves from the King in Yellow. At the end of the play, the Stranger, now called Haita reveals that that which appears to be a mask is in fact his true face.
When the King makes his appearance at the end of the play, and overwhelms Haita, he curses the people of Yhtill and tells them that the price of victory over Alar shall be 'the fixing of the mask'.