Plans: Sageblade

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

It has also been reported to drop (vanishingly rarely) from random elite mobs; and less infrequently is found in Eternium Lockboxes.