Death Mask is a Legendary Helm in Diablo III, only available as part of the Arma Haereticorum bundle if one purchases the Collector's Edition (or Digital Deluxe edition) of Reaper of Souls. It can only be used for Transmogrification.
"I argue that humans, not angels, are the true inheritors of Anu. It was only when the god rejected the part of himself that all mortals possess, the darkness within, that true evil and suffering was born. Unlike Anu, we must embrace our inner demons... and become all the stronger for it." — Zoltun Kulle