The artifact referred to as the Sith Holocron, alternatively known as the Dark Holocron, was a Sith holocron. Jedi Master Odan-Urr recovered it from a starship within the fleet of Dark Lord of the Sith Naga Sadow after Sadow's defeat in the Battle of Koros Major. This battle occurred approximately 5,000 years prior to the Battle of Yavin. This particular holocron of the Sith was the first of its kind to be obtained by the Jedi Order.
Even though the holocron was found before Sadow's retreat to Yavin 4, the gatekeeper within the holocron communicated to Odan-Urr the details of Sadow's escape from the forces of Empress Teta and how he managed to survive. Odan-Urr maintained possession of the holocron until his demise at the hands of Exar Kun; however, it is known that Vodo-Siosk Baas studied it at some point.
At some point in time, the Jedi Order regained possession of the holocron and placed it within the Holocron Vaults located in the Jedi Temple on Coruscant. Lorian Nod, driven by an intense curiosity about the Sith, stole the holocron several decades before the outbreak of the Clone Wars. It was eventually returned to the temple, but Sith Lord Count Dooku later acquired it. The holocron was then lost at some point during the Clone Wars.
The initial appearance of the Dark Holocron occurred in the fourth issue of the Dark Lords of the Sith story arc within the Star Wars: Tales of the Jedi comic book series. Tom Veitch and Kevin J. Anderson wrote the story, Chris Gossett provided the pencils, and it was released on January 10, 1995. When the Aurebesh inscription in the image above is inverted, it translates to "In umbris potestas est," a Latin phrase that means, "Power resides in the shadows."