The Mask of Sliske, Shadow, also known as simply Shadow, is a sentient mask that is involved in the Missing, Presumed Death quest and features as a reward.
Shadow is first encountered during the quest, when it is guarding the Shadow Chamber in the Empyrean Citadel for its master, Sliske, with its companion, Light, who it hates and describes as a "brainless moron". Shadow is very grumpy and depressed, and after the puzzle is completed, begs you not to leave him there.
It is given as a reward after the quest by Brother Samwell, and it can be worn, be spoken to while in inventory, and switched to Light. If lost, it can be reclaimed from the Statue of Death in the citadel. Upon reclaiming, it will be removed from inventory, bank, equipment or even keepsake box if one is owned.
Shadow has a "Talk to" option while in your inventory. This will cause the mask to say one of the following lines: