The Codex of Tython was a Jedi holocron in the shape of an icosahedron. It was rumored to contain the oldest and most guarded Jedi secrets.
The Codex of Tython was discovered on Dantooine by a Rebel agent, when his landspeeder crashed into a cache of the hidden Jedi relics, among them the Codex. The Holocron recorded thousands of years of technological accomplishments, and the agent became determined to deliver the Holocron to the Alliance. The discovery, however, became known to the Galactic Empire, who wanted to obtain the Holocron as well. On his way back to the spaceport the Rebel was approached by Captain Adrick and his stormtroopers. Adrick asked the Rebel to hand over the Codex, and when the latter refused, Adrick ordered his men to kill him. The agent was only saved by a Jedi named Drakka Judarrl, who created an illusion, which allowed him to escape. The Zabrak Jedi explained that the Codex was missing some fragments, which had to be re-inserted into the Holocron, before it can be operational again. She provided one fragment herself, the other three the Rebel was able to obtain at great risk from a Dark Jedi Mellichae, Nightsister Axkva Min and Grand Admiral Andal Sait. Finally reassembled, the Codex was given to Drakka Judarrl for study, and several Alliance agents stayed with her to guard it.
This achievement, however, was short-lived. An ISB agent looking for the Codex came to Drakka and was able to take it from her. As an archaeologist experienced with such ancient recording devices, he was able to examine the holocron and activate it. The object flashed with many images of all kinds of worlds and cultures; the Imperial, however, was interested by the plans for a massive device that moved asteroids. Evidently the Jedi planned to use this tractor beam for mining purposes, placing ore-bearing planetoids in easier reach for advanced civilizations to construct skyscrapers and power plants. The ISB agent, however, was more interested in military applications of this asteroid mover. Such a device could crush a starship with its power or be used to bombard a planet with asteroids. He contacted Grand Admiral Andal Sait, showing him the plans for the superweapon. Sait became intrigued and asked the agent to deliver the Holocron to him, along with the a focusing crystal needed for the weapon. The agent performed the task, which allowed Imperial engineers to install the superweapon aboard Blackguard, Sait's Imperial-class Star Destroyer.