Part of the Finery of Infinite Wisdom set:

The set gives these bonuses as more pieces are worn: 3: Increases the damage of your Shadow Word: Pain spell by 5%. Set Details | All Sets

Quest Reward from:

Level	Name	Start	Rewards
60	Shroud of Infinite Wisdom  (Raid) Horde and Alliance Ahn'Qiraj	Keyl Swiftclaw Silithus	6600 XP