One of the most devastating events in Star Wars history was the Great Sith War, in which the Jedi Exar-Kun turned to the dark side and declared war against both the Old Republic and the rest of the Jedi. Soon, other factions were pulled into the fight, and devastation spread across the Galaxy.
Sometimes referred to as the Exar Kun War, the conflict saw the Dark Jedi rise to become the greatest lightsaber duelist who ever lived. It also caused whole planets to fall and had repercussions that would be felt through the Galaxy for millennia.
Setting the Scene
The tragic events of the Great Sith War were depicted in Tales of the Jedi, a Legends comics series whose stories took place during the Old Republic Era (and earlier). Millennia before the war, the ancient Jedi Order split with followers of the dark side of the Force. The Dark Jedi went into exile and discovered the alien Sith, with whom they joined forces and intermarried. The first Sith Lord was entombed alongside the kings of the alien Sith on Korriban.
Later, a Sith half-breed and powerful alchemist named Naga Sadow became Dark Lord, building a temple on Yavin IV, where he trained the next Sith Lord, Freedon Nadd. After killing his master, Nadd conquered the planet Onderon, making himself king until he was slain by the Jedi. His spirit returned just ten years before the Great Sith War in an event known as the Freedon Nadd Uprising, which included many of the primary characters of the coming war.
The Sith Brotherhood
Exar Kun was a Jedi whose lightsaber skill and thirst for knowledge were unparalleled. After deciding the Jedi were keeping him from reaching his potential, he traveled to Korriban, where the ghost of Freedon Nadd mentored him. He then went to Yavin IV and took control of an army of Massassi (aliens descended from the ancient Sith) who Naga Sadow had enslaved there.
Meanwhile, the Jedi Ulic Qel-Droma--a hero of the Freedon Nadd Uprising--tried to infiltrate the dark side group called the Krath. Qel-Droma was seduced by Aleema Keto, one of the Krath's leaders, who lured him to the dark side. He killed Aleema's cousin and took his place beside her. He turned his back on the Jedi, including his brother Cay. Perceiving the Krath to be a threat, Exar Kun attacked them but ended up forging a pact with Ulic Qel-Droma, creating the Brotherhood of the Sith. Exar Kun became the new Sith Lord with Qel-Droma serving beside him.
The Great Sith War
Together, the two Sith attacked the Republic. Ulic Qel-Droma defeated Mandalore the Indomitable in single combat, thereby recruiting the Mandalorians to his cause. Meanwhile, Exar Kun destroyed an ancient Sith Holocron, using its shards to corrupt other Jedi, thereby gaining an army of Dark Jedi.
Qel-Droma attacked the planet Coruscant but was captured. Exar Kun sent his fallen Jedi to kill their masters while he rescued Qel-Droma, storming into the senate chambers in the middle of the trial and killing both the Supreme Chancellor and his former master with his newly-constructed double-bladed lightsaber. The surviving Jedi (including Ulic's brother, Cay Qel-Droma, and the legendary Nomi Sunrider) gathered at the Great Jedi Library on Ossus. The Sith Lords invaded Ossus, where Ulic Qel-Droma killed his brother Cay in a duel. As Ulic fell to his knees with regret, Nomi Sunrider ripped away his connection to the Force.
Ossus and the Jedi Library were destroyed when the Sith made multiple stars go supernova. Exar Kun escaped, but Ulic--filled with remorse--led the remaining Jedi to Yavin IV, where Kun was using Sith alchemy to transcend his physical body and achieve immortality. Countless Jedi countered his ritual with the light side of the Force, defeating Kun and trapping him as a formless spirit in the temple, even as his remaining armies were defeated and his last generals slain.
Aftermath
The effects of the Great Sith War would be felt for generations. The Jedi Academy moved to Coruscant, while the Sith Brotherhood established an academy on Korriban, which helped the Sith Lord Revan train an army to attack the Old Republic. Much later, Darth Maul based his own double-bladed lightsaber on Exar Kun's. When the Rebel Alliance destroyed the Death Star in the Battle of Yavin, they were using the old Sith temples for bases.
Ten years later, Luke discovered a trove of lightsabers on Ossus from the old Jedi Library. After Luke reformed the Jedi Order, his students encountered Exar Kun's ghost on Yavin IV, who tried tempting them to the dark side. But the ghost of Kun's old master also returned, banishing the Sith Lord once and for all.