Part of the Cryptstalker Armor set:

The set gives these bonuses as more pieces are worn: 2: Increases the duration of your Rapid Fire by 4 secs. 4: Increases Attack Power by 50 for both you and your pet. 6: Your ranged critical hits cause an Adrenaline Rush, granting you 50 mana. 8: Reduces the mana cost of your Multi-Shot and Aimed Shot by 20. Set Details | All Sets

Disenchants into

Name	Value	Drop%
Nexus Crystal	1 Copper	100.0%