Part of the Gift of the Gathering Storm set:

The set gives these bonuses as more pieces are worn: 3: Increases the chain target damage multiplier of your Chain Lightning spell by 5%. Set Details | All Sets

Quest Reward from:

Level	Name	Start	Rewards
60	Hammer of the Gathering Storm  (Raid) Horde and Alliance Ahn'Qiraj	Warden Haro Silithus	6600 XP