With some work you can turn in the Libram of Tenacity to Mathredis Firestar to receive a Lesser Arcanum of Tenacity, which will permanently add 125 armor to your own leg or head slot item. Applying the Arcanum will make the item soul bound. You must be Level 50 or above to apply the Arcanum.
Drops off level 50 and above humanoids or undead.
<The pages are covered in ancient elven runes.>
The pages herein contain memories of events that transpired in the collection and creation of the reagents required to craft lesser arcanum.
May our enemies never gain access to these libram.
May I live to see the pallid light of the moon shine upon Quel'Thalas once again.
May I die but for the grace of Kael'thas.
May I kill for the glory of Illidan.
As of patch 3.1, Lesser Arcanum can ONLY be applied to your own head or leg item. Applying them causes the item to become soul bound (tested on Commander's Helm of Defense) "Enchanting this item will bind it to you". You can no longer apply them through a trade window (tested) "Can only be applied to your own items". They require Level 50.