Plans: Mithril Shield Spike

Source
This item is a world drop from creatures of levels 42-52 (approximately).

It has also been noted to drop (at non-negligible rates) from the following:
 * 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