During the Cold War, Darth Rictus was head of the Sphere. Due to the secretive, and often bloody, work of the sphere, the upper echelons of Imperial society nicknamed the organization the Assassin's Pyramid. Of all the Spheres of Influence, the Sphere of Mysteries was the most covert and secretive. As a result, most citizens of the Empire were not even aware of what the Sphere's true responsibilities were.
After the Empire's defeat at the hand of the Eternal Empire, this sphere was combined with the Spheres of Ancient Knowledge and Sith Philosophy into the Sphere of Sith Doctrine.
- Star Wars: The Old Republic Encyclopedia