The Mask is an important symbol to the Satanists.

Seems to represent the "channeling" or summoning of dark spirits, to conjure some power.

The earliest recorded uses of the mask, outside of secret rituals, is the "Harlequin" mask from Medieval theater, which actually is derived from much more ancient iconography than that, going all the way back to the mask of the Wild Man or Green Man pagan demon.

Harlequin is the name we use today, derived from Hellekin and Arlecchino. Interesting that the Harlequin masks became so intimately associated with theater, which evolved in the last century into film. The concept of performance and c asting a spell over an audience's imagination seems to be really central to the Satanist's magick.

From "Stages of Evil":

Consider the following apt description of the curious animalistic half mask worn by Arlecchino since the inception of the stage character in the sixteenth century: “I found myself staring, for the first time, into the pin-sized eyeholes of Arlecchino’s early mask: . . . the sly brutish features, the two flamboyant warts, the animal hair of this dark leather face. . . . It was a shock to meet . . . the feral ancestor of the shining Harlequin”