A mirror shield is a piece of slayer equipment used for fighting the cockatrice and basilisk slayer monsters, requiring 25 Slayer and 20 Defence to wield. When fighting these monsters, the shield must be equipped, or else the player's stats will be drastically reduced. It can be bought from any Slayer master, or by trading with another player.
Interestingly, the shield's image moves with the camera's angle to appear like a reflection when either equipped or dropped on the ground, but the shield shows the same image no matter where it is at. The shield appears to have low luster because of its grey colouration.
Mirror shields are good alternatives to elemental shields, if players haven't finished the quest Elemental Workshop I. They also provide a +5 bonus in magic defence, making the shield useful not only for cockatrices and basilisks, but for magic-based attacks as well.