• Antediluvian Cornerstone Grimoire
• Item Level 245Disenchants into:
  [Abyss Crystal]: 1-2
• Binds when picked up
• Held In Off-hand
• +51 Stamina
  +66 Intellect
  +46 Spirit
• Requires Level 80
• Equip: Increases spell power by 75.
  Use: Summons a Skeleon that will protect you for 1 min. (15 Min Cooldown)
• Sell Price: 7 54 52

Source

This item drops from Onyxia in the 25-man version of Onyxia's Lair.

Patch changes

• Patch 3.2.2 (2009-09-22): Added. 
  

External links

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