Netherwind Crown (tier 2)

Netherwind Crown is part of the Netherwind Regalia, the raid Tier 2 armor set for Mages.

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.