Hallowed Helm 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.
"Years ago, one of my subordinates went missing in Caldeum. It was presumed by most that he'd run afoul of a demonic ambush. I myself suspected Zoltun Kulle's involvement in the disappearance, as his egotism and madness had begun to evince itself in his imperious and dismissive manner. I suppose we’ll never know the truth." — Archangel Tyrael