I think this is the mastery for Spawn related powers. I think it improves the damage of clawed strike and possibly the chance to hit well with tentacles. But I don’t have any good info on this.
The ingame help reads:
Skill duration: random. Type of skill : mastery skill. Affecting stats: None. It uses no endurance points. Some followers of Azzarakk have developed killing and mutilation into a chaotic form of art. While these followers are rare, the discipline is terrible to behold.