The Mask of the Wraith is an object that allows the user to become The Sandwraith, another aspect of him/herself, to defy fate.
When the Prince kills Kaileena, the Empress of Time, he unleashes the Sands of Time whose creation he was trying to prevent. Just as he was about to lose hope, he learns of the Mask that could help him cheat fate. Putting it on, he becomes the Sandwraith, the creature he saw all across the Island of Time. But realizing what he had done to create the Sands, he allows his "other" self to be killed by the Dahaka. This not only prevents the creation of the Sands, but also allows him to remove the mask. By no other means can the mask be removed unless the users "other" self dies so that the wearer of the mask takes his place in the timeline.