Nemesis Skullcap (tier 2)

The Nemesis Skullcap is part of the Nemesis Raiment, the raid Tier 2 armor set for Warlocks.

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.