Eye of C'Thun

Eye of C'Thun is the proof of the Old God C'Thun's defeat.

Source
This item always drops from C'Thun, the final boss of the Temple of Ahn'Qiraj.

Chain Quest
The Eye of C'Thun begins a short chain quest:
 * Quest:C'Thun's Legacy
 * Quest:The Savior of Kalimdor

You get to choose one of the following at the end of the quest chain:

Note
Eye of C'Thun is also the name of C'Thun's gigantic eyeball form in phase 1. However, the item is described as "an eye stalk of the fiend", which is likely the eye of a tentacle.