Mantle of the Avatar

This item is part of the Avatar Raiment armor set, the Tier 5 raid set for healing priests.

Source
This item is sold by in the Seer's Library on the Scryer's Tier and by  in the Shrine of Unending Light on Aldor Rise in Shattrath City. The cost of the item is a Pauldrons of the Vanquished Defender token.