The hollow mask in the Japanese manga series "Bleach" is the white, skull-like mask worn by a hollow, a monstrous creature born from a human soul that has stayed in the human world and failed to cross over to Soul Society. The mask represents the hollow's lost heart.
The mask obscures the identity of the hollow as a former human being and thwarts its humane instinct after the loss of its heart. The mask can shatter temporarily, which allows the hollow to remember its original identity, but the mask usually regenerates and resumes protecting the hollow's naked instinct. The powers a hollow acquires are the manifestations of the soul's heart.