Power awaits within.
—An etching on the edge of the Atrum
The Atrum was a Sith Holocron forged by an unknown Sith Lord during the Sith Wars. It was imbued with an incredible amount of raw Dark Side power, making it an incredible energy source. Intending to make sure that the power of the Atrum was exclusive to himself only, the unknown Sith Lord placed it on a starship and programmed it to head towards a random planet, confident that he would be able to locate it using the Force. Just to make sure, he left a series of clues across the galaxy that would eventually lead to its location, starting with a secondary holocron he created. However, he was killed by his own power-hungry aides, and the Atrum remained hidden for thousands of years, eventually becoming a legend in some parts of the galaxy.
Eventually, however, around 0 BBY, the secondary holocron that started off the series of clues was discovered by an archaeological team on Thule. Palpatine, intrigued by this new discovery, ordered Darth Vader himself to study this, aroused by the possibility of locating the legendary Atrum. However, before he could arrive, agents from the Rebel Alliance and Black Sun turned up the site of the discovery, both intent on getting away with the holocron. After a fierce battle, the rebels made off with the holocron, with the Black Sun minions in hot pursuit. Unbeknownst to both of them, the archaeologists had made copies of the etchings on the Holocron, so Vader had something to work from when he arrived.
Very soon, an epic galaxy-wide chase began, with the Atrum as the ultimate prize.
Behind the scenes
'Atrum' is Latin for 'black'.