Eye of the Black Prince

Eye of the Black Prince adds a prismatic socket to any Sha-Touched weapon.

In Patch 5.2, an Eye can be attached to any weapon that drops from the Throne of Thunder, in addition to the previous nine sha-touched weapons.

Source
This item can be purchased from Commander Oxheart in Townlong Steppes and Blacktalon Quartermaster at the Veiled Stair for by players who have completed the following quests: Before patch 6.0.2 this was also a reward from the above quests, but they have since been removed.
 * Call of the Packmaster
 * The Soul of the Horde