The Sphere was a construct that became pivotal in the closing stages of 128 ABY, the second year of the Sith-Imperial War. Created by Darth Atavus in 80 BBY, the Sphere was used to draw the Galactic Alliance and the Jedi Order into a trap that became known as the Battle of Roon.
Key to Hoole's plans were the Xenly - Dark Side-corrupted Verpine who lived, seemingly, to create more darkness. In attempting to harness them, Hoole made efforts to construct what was then a tower on Roon, where a particularly large hive of Xenly remained. The construction continued even as Hoole aged, but at the center of this impossibly massive complex was no heart, nor mind. The mind would be Hoole, when he used the Sphere to bind the darkness of the Verpine together, in a way none to dissimilar to how Lomi Plo bound together the Killiks by becoming a Joiner.
Hoole was forced into stasis to maintain his life, and left the Xenly to run the Sphere. He emerged from stasis in an effort to control it, but was unable too - the Sphere had no heart, and Hoole was at a loss how to create one.
Stymied, Hoole returned to stasis, even as the One Sith took Roon as their second capital world. They discovered the abandoned tower, and believed it to be capable of harnassing the natural energy of the dark side world, entirely missing the Xenly or their significance. They experimented with the tower secretly, carefully, until the dark side genius Darth Wyyrlok II concluded in 105 ABY that placing a Sith Meditation Sphere within the top of the base. The tower was built around it, continuing, eventually the Meditation Sphere within it being tiny in comparison to what was connected to it.
The Sphere grew in power, entrenching itself in the world that was Roon, becoming unmistakably real, if not alive.
But the Sphere wouldn't respond to the One Sith, for the Xenly behind it were, in their own way, loyal to Hoole - their master, their friend. Only Hoole could control the Sphere - but only through Sith teachings. And so, when the two parties finally met, Hoole and the One Sith did the obvious thing; they joined forced.
Hoole became Darth Atavus, the first Fist of the One Sith.
Darth Krayt carefully, personally, taught Hoole his secrets, and Hoole, for the first time in his life, served. He grew more powerful than he had ever been as a Dark Jedi, joining the elite of the One Sith, working beside those who made up the most dangerous and cunning Sith in the history of the order.