Wikia

The Sith mask was used during the days of the Jedi Civil War by some Sith to block outside influences and allow the user to focus without distraction. Many sith masks appeared to lack eye openings or otherwise restricted the vision, presumably requiring the wearer to perceive his or her surroundings through the Force. The Ysanna, a band of Force-users from Ossus, also used Sith masks in combat, wearing them to frighten their enemies.

Another type of Sith mask was once worn by an unknown Sith Lord. It was later found by an unknown spacer in 1 ABY.^[1]