The Shadow Sash is an ancient sash worn by each of the "Sisters" of the Brotherhood of Shadows. The sash first appeared with Quan Chi's old assassins Kia, Jataaka and even Sareena in Mortal Kombat Mythologies: Sub-Zero. It later reappeared as a collectible Relic called Sareena's Sash in the Konquest mode of Mortal Kombat: Armageddon.
Worn once by Sareena, former denizen of the Netherrealm and mysterious former servant of the great demon sorcerer Quan Chi, this sash displays the symbol of the Brotherhood of Shadow, the dark and ancient society of faithful worshipers of the fallen Elder God known as Shinnok.
Sareena, along with her fellow demons Kia and Jataaka, served as Quan Chi's enforcers and full-fledged members of the Brotherhood for many years until she defected from his control thanks to the human once known as the original Sub-Zero and helped defeat her former master. And because of that ordeal, her master's deity Shinnok punished her for the act of betrayal.
More sisters of the Brotherhood of Shadows who wore these same Shadow Sashes are Siann, Mika and Sora, who also served the sorcerer Quan Chi, who is part of the Brotherhood of Shadows in Mortal Kombat: Conquest.
Quan Chi himself sometimes wears a similar sash, though his lacks the Brotherhood of Shadows emblem on the front.