A significant conflict unfolded on Ossus in 3996 BBY, known as a battle during the Great Sith War. It stands as one of the concluding major engagements of that era. Aleema Keto triggered the destruction of the Cron Cluster using Sith magic, resulting in a massive supernova hurtling towards the Jedi library world of Ossus. Recognizing that this Sith-induced supernova would necessitate a Jedi evacuation of Ossus, the Sith Lords Exar Kun and Ulic Qel-Droma orchestrated an assault on the Jedi planet. Their objective was to seize the immense repository of Force knowledge and ancient texts accumulated over millennia. Leading a force comprised of Mandalorian warriors and their Krath army, Qel-Droma and Kun launched their attack on Ossus, capitalizing on the disarray and pandemonium caused by the impending cataclysm.
In the aftermath of their initial invasion, both Sith Lords became embroiled in various smaller conflicts. Ulic eventually faced his brother Cay, who sought to redeem Ulic and bring him back to the light side. Their confrontation culminated in a duel where Ulic killed Cay Qel-Droma. Nomi Sunrider, witnessing Cay's death, used her abilities to sever Ulic's connection to the Force. Finally understanding the extent to which he had been manipulated by the dark side, Ulic expressed remorse and offered to guide the Jedi to Kun's hidden stronghold on the jungle moon of Yavin 4.
During the Battle of Kemplex Nine, an ancient Sith device, tampered with and used to manipulate matter within the cores of stars, caused the detonation of the nearby stellar group known as the Cron Cluster. This triggered a massive supernova shockwave that was headed directly for the Jedi scholar world of Ossus.
A widespread planetary evacuation was initiated, with military, civilian, and Jedi personnel alike struggling to escape the planet before the arrival of the nova-wave. The Jedi Masters present on the planet began a rescue operation, focusing on salvaging as many relics, manuscripts, and other antiquities as possible from the enclave while time remained. This opportunity proved too tempting for Exar Kun to ignore. Joining forces with fellow Sith Lord Ulic Qel-Droma and reinforcements of Mandalorian and Massassi warriors, and diverting Lord Mandalore the Indomitable's main battle group to Onderon, Kun launched a raid on Ossus itself, intending to capture the vast amounts of Jedi knowledge stored there.
As the evacuation reached critical levels, the Sith and Mandalorian ships converged on the planet. They targeted numerous civilian and military transport craft and engaged the Jedi defenses long enough for Kun to slip through the defenses and land in the enclave. Among his primary targets was the residence of Master Ood Bnar, who was attempting to preserve a collection of antique Jedi lightsabers from the impending destruction. Engaging Kun and his Massassi, Master Bnar realized that his true strength lay not in combat, but in the powers of the Force. Drawing upon it, the Neti Jedi Master sealed the cache, transforming into a great tree and thwarting the incredulous Dark Lord. He remained planted there until awakened during Luke Skywalker and Kam Solusar's mission to Ossus in 10 ABY.
Nearby, the Cathar Jedi Sylvar attacked Kun near his ship, seeking revenge for the death of her mate Crado's at Kemplex Nine, but was defeated by his elite Massassi guard.
Meanwhile, Cay Qel-Droma sensed his brother Ulic's presence among the battling orbital and sub-orbital ships, and took the Nebulon Ranger to the skies in an attempt to persuade Ulic to return to the light. Shot down by Mandalorian ships during the melée, Cay was forced to crash-land in the city, where the two brothers confronted each other for the final time. Attempting to reason with him, Cay only succeeded in provoking Ulic's temper. The dark side surged within him, and Ulic initiated a violent lightsaber duel, severing his younger brother's mechanical arm. Pleading with him, Cay tried to reach the Jedi deep within Ulic…but in a final surge of anger, Ulic killed his brother where he knelt.
As a final act of punishment, Nomi Sunrider imprisoned Ulic behind a wall of light side Force-energy. Ulic was forever cut off from the Force and captured by the Republic, thereby becoming the battle's last casualty. The stage was now set for the final showdown with Exar Kun on Yavin IV.
The Battle of Ossus initially appeared in the Tales of the Jedi comic series, specifically in the issue The Sith War 5: Brother Against Brother. The battle was conceived and written by author Kevin J. Anderson, with artwork by Dario Carrasco, Jr. The cover art for Brother Against Brother, depicting a duel between Ulic and Cay Qel-Droma, was created by Hugh Fleming, a veteran Tales of the Jedi artist.
Anderson used the Battle of Ossus to conclude Ulic Qel-Droma's reign as a Sith Lord and to continue the chain of events, initiated during the writing of the Battle of Kemplex Nine, which would lead to the fall of Exar Kun and the end of the Great Sith War.