• Arcanum of Torment
• Item Level 80Disenchants into:
  Not disenchantable
• Binds to Battle.net account
• Requires Knights of the Ebon Blade - Revered
• Use: Permanently adds 50 attack power and 20 critical strike rating to a head slot item. Does not stack with other enchantments for the selected equipment slot.
• Sell Price: 37 50
• Vendor 150

Source

This item is a reputation reward; you must be revered with Knights of the Ebon Blade to buy it from Duchess Mynx <Ebon Blade Quartermaster> for 150.

Patch changes

Patch 3.3.0 (08-Dec-2009): Top-level helm and shoulder faction-related enchants are now available as Bind-on-Account items that do not require any faction to use once purchased (they still require the appropriate faction level to purchase). 

External links

• Battle.net (EU)
• Wowhead
• Thottbot
• WoWDB