Halo of Transcendence (tier 2)

Halo of Transcendence is part of the Vestments of Transcendence, the raid Tier 2 armor set for Priests.

Source
Prior to the release of patch 3.2.2, this item dropped from the level 60 version of Onyxia. It has since been moved to Nefarian in Blackwing Lair, along with the rest of the tier 2 helms.