Part of the Demoniac's Threads set:

The set gives these bonuses as more pieces are worn: 2: Increases damage and healing done by magical spells and effects by up to 12. 3: Increases the damage of Corruption by 2%. 5: Decreases the cooldown of Death Coil by 15%. Set Details | All Sets

Quest Reward from:

Level	Name	Start	Rewards
60	Paragons of Power: The Demoniac's Robes  (Raid) Horde and Alliance Zul'Gurub	Al'tabim the All-Seeing Stranglethorn Vale	6600 XP