Plans: Helm of the Great Chief

Source
This item drops from:
 * Onyxia in Onyxia's Lair
 * Lord Kazzak in Blasted Lands
 * Emeriss in Ashenvale
 * Lethon in Ashenvale
 * Highlord Kruul in Silithus
 * Azuregos in Azshara
 * Nefarian in Blackwing Lair
 * Taerar in Ashenvale
 * Ysondre in Ashenvale

It has also been found in Eternium Lockboxes.