Shadow Trance
Part of Soul of the Malevolent.
Permanently imbue your shoulders with the Soul of the Malevolent: Periodic damage from your Shadowflame, Unstable Affliction, and Curse of Agony spells and damage done by your Felguard have a 4% chance to grant the Shadow Trance effect.
Data from the WoW Forever client, build 1.60.1.70862, extracted .