The Council of the Black Harvest (previously called "The Council of Six Daggers" in the Beta) is a powerful, shadowy organization of 6 warlock trainers from around Azeroth and Outland whose existence was revealed by the book, [Legacy of the Masters]. They have presumably "unlocked the secrets of the most destructive forces to ever threaten Azeroth." What this means is not immediately clear, but implies that this group has discovered some new powers in the wake of Deathwing's fall.