This story is collected in:

Tales of the Jedi: Volume 2

Comic Book series published August 1995 thru January 1996 as Tales of the Jedi: The Sith War #1, 2, 3, 4, 5 & 6 by Dark Horse Comics:

Issue #1Issue #2

Issue #3Issue #4

Issue #5Issue #6

Synopsis:
Picking up six months after Dark Lords of the Sith, Exar Kun and Ulic Qel-Droma are implementing their insidious plan for revitalizing the Golden Age of the Sith. While insinuating his darkside teachings to Jedi students on the planet Ossus, it becomes clear that Kun is only concerned with securing a powerful Sith Holocron from its sealed repository in the great library of Ossus. Ulic has devoted himself to developing the darkside military strategy, the key to which may just be winning the loyalty of the ferocious warrior race led by a masked mysteryman named Mandalore.
Ulic Qel-Droma spent years learning to control the Force. He wanted to be on the side of the good, a member of the Jedi elite. But it is not the Force one must control to become a Jedi; rather, one must learn to control oneself. Ulic Qel-Droma has tasted power, and now he cannot fight the dark hunger inside him . . .
Ulic Qel-Droma has fallen victim to the spell of the dark side, and now he must stand before the Jedi masters to answer for his crimes. But another from within the Jedi ranks has taken up his mantle of evil. Exar Kun is turning his doting apprentices into Sith-possessed assassins and sending them out to kill the Jedi masters, and the fate of the entire Jedi force hangs in the balance!
The deadly Jedi assassination pogrom has begun! It's Jedi against Jedi as Exar Kun and his young followers, "the Brotherhood of the Sith," have risen up against their former Jedi Masters. Sorely outnumbered, only a miracle could save them and a miracle is what they get! The legendary Jedi knights, Cay, Nomi Sunrider, and Sylvar stand between the Brotherhood and the Jedi Masters, and neither side will quit until the other has fallen!
It's Jedi against Jedi, brother against brother, and blood will spill. Brothers Cay and Ulic Qel-Droma lead their respective Jedi forces against one another: Cay with his honorable but unproven students, Ulic with the ascendant Dark Jedi horde. As the forces clash on the planet Ossus, the right to the powerful historical artifacts of the ancient Jedi await the victor.
The storm clouds of battle gather over Yavin Four, and that entire world could be a casualty! To make his final stand against Jedi Nomi Sunrider, the betrayer Ulic Qel-Droma has let slip the dogs of war in a final, climactic confrontation.

Synopsis taken from Dark Horse Comics, www.darkhorse.com.