"The Sith have possessed the Syn Holocron since the time of Bane—the one in the Temple Archive was a spying device concealed with a shrouding spell."
Darth Sidious[src]

The Syn Holocron was a Sith holocron that was constructed by the Sith Lord Sorzus Syn near to the end of her life. The holocron was reacquired by the Sith thousands of years later, during the time of the Sith Lord Darth Bane, and the Order of the Sith Lords possessed the device during the waning years of the Galactic Republic. At some point, the Jedi Order recovered an object that they believed to be the Syn Holocron, but it was in fact a spying device that was disguised by a shrouding spell.